Here is the troubleshhooting information:

 Application Basics

   Name: Thunderbird
   Version: 91.3.2
   Build ID: 20211117150618
   Distribution ID:

   Update Channel: release
   User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.3.2
   OS: Windows_NT 10.0 19043

   Launcher Process: Enabled
   Multiprocess Windows: 0/0
   Fission Windows: 0/0
             Disabled by default
   Remote Processes: 3
   Enterprise Policies: Active
   Google Location Service Key: Missing
   Google Safebrowsing Key: Missing
   Mozilla Location Service Key: Missing
   Safe Mode: false

 Mail and News Accounts
   account2:
     INCOMING: account2, , (none) Local Folders, 0, passwordCleartext

   account5:
     INCOMING: account5, , (pop3) pop.centurylink.net:995, SSL, passwordCleartext
     OUTGOING: , smtp.centurylink.net:587, alwaysSTARTTLS, passwordCleartext, true

 Calendar Settings (not used - deleted due to posting size limit)   

 Crash Reports for the Last 3 Days

 Remote Processes

   Type: Count

     Web Content: 1 / 8
     Extension: 1
     GPU: 1

 Add-ons

     Name
     Type
     Version
     Enabled
     ID

       Amazon.com
       extension
       1.1
       true
       amazondotcom@search.mozilla.org

       Bing
       extension
       1.0
       true
       bing@search.mozilla.org

       Bitdefender Toolbar
       extension
       24.22.0.13
       true
       bdThunderbird78@bitdefender.com

       DuckDuckGo
       extension
       1.0
       true
       ddg@search.mozilla.org

       Google
       extension
       1.0
       true
       google@search.mozilla.org

       Wikipedia (en)
       extension
       1.0
       true
       wikipedia@search.mozilla.org

       Show All Body Parts
       extension
       1.3.6
       false
       ShowAllBodyParts@kamens.us

 Security Software

   Type: Name

     Antivirus:
     Antispyware:
     Firewall:

 Graphics

     Features
     Compositing: WebRender (Software)
     Asynchronous Pan/Zoom: wheel input enabled; scrollbar drag enabled; keyboard enabled; autoscroll enabled; smooth pinch-zoom enabled
     WebGL 1 Driver WSI Info: EGL_VENDOR: Google Inc. (adapter LUID: 0000000000011413)

EGL_VERSION: 1.5 (ANGLE 2.1.14226 git hash: 31a43497be09) EGL_EXTENSIONS: EGL_EXT_create_context_robustness EGL_ANGLE_d3d_share_handle_client_buffer EGL_ANGLE_d3d_texture_client_buffer EGL_ANGLE_surface_d3d_texture_2d_share_handle EGL_ANGLE_query_surface_pointer EGL_ANGLE_window_fixed_size EGL_ANGLE_keyed_mutex EGL_ANGLE_surface_orientation EGL_ANGLE_direct_composition EGL_NV_post_sub_buffer EGL_KHR_create_context EGL_EXT_device_query EGL_KHR_image EGL_KHR_image_base EGL_KHR_gl_texture_2D_image EGL_KHR_gl_texture_cubemap_image EGL_KHR_gl_renderbuffer_image EGL_KHR_get_all_proc_addresses EGL_KHR_stream EGL_KHR_stream_consumer_gltexture EGL_NV_stream_consumer_gltexture_yuv EGL_ANGLE_flexible_surface_compatibility EGL_ANGLE_stream_producer_d3d_texture EGL_ANGLE_create_context_webgl_compatibility EGL_CHROMIUM_create_context_bind_generates_resource EGL_CHROMIUM_sync_control EGL_EXT_pixel_format_float EGL_KHR_surfaceless_context EGL_ANGLE_display_texture_share_group EGL_ANGLE_display_semaphore_share_group EGL_ANGLE_create_context_client_arrays EGL_ANGLE_program_cache_control EGL_ANGLE_robust_resource_initialization EGL_ANGLE_create_context_extensions_enabled EGL_ANDROID_blob_cache EGL_ANDROID_recordable EGL_ANGLE_image_d3d11_texture EGL_ANGLE_create_context_backwards_compatible EGL_KHR_create_context_no_error EGL_KHR_reusable_sync EGL_EXTENSIONS(nullptr): EGL_EXT_client_extensions EGL_EXT_platform_base EGL_EXT_platform_device EGL_ANGLE_platform_angle EGL_ANGLE_platform_angle_d3d EGL_ANGLE_platform_angle_d3d11on12 EGL_ANGLE_device_creation EGL_ANGLE_device_creation_d3d11 EGL_ANGLE_experimental_present_path EGL_KHR_client_get_all_proc_addresses EGL_KHR_debug EGL_ANGLE_explicit_context EGL_ANGLE_feature_control IsWebglOutOfProcessEnabled: 1

     WebGL 1 Driver Renderer: Google Inc. -- ANGLE (NVIDIA GeForce GT 1030 Direct3D11 vs_5_0 ps_5_0)
     WebGL 1 Driver Version: OpenGL ES 2.0.0 (ANGLE 2.1.14226 git hash: 31a43497be09)
     WebGL 1 Driver Extensions: GL_ANGLE_base_vertex_base_instance GL_ANGLE_client_arrays GL_ANGLE_depth_texture GL_ANGLE_explicit_context GL_ANGLE_explicit_context_gles1 GL_ANGLE_framebuffer_blit GL_ANGLE_framebuffer_multisample GL_ANGLE_get_tex_level_parameter GL_ANGLE_instanced_arrays GL_ANGLE_lossy_etc_decode GL_ANGLE_memory_size GL_ANGLE_multi_draw GL_ANGLE_multiview_multisample GL_ANGLE_pack_reverse_row_order GL_ANGLE_program_cache_control GL_ANGLE_provoking_vertex GL_ANGLE_request_extension GL_ANGLE_texture_compression_dxt3 GL_ANGLE_texture_compression_dxt5 GL_ANGLE_texture_usage GL_ANGLE_translated_shader_source GL_CHROMIUM_bind_generates_resource GL_CHROMIUM_bind_uniform_location GL_CHROMIUM_color_buffer_float_rgb GL_CHROMIUM_color_buffer_float_rgba GL_CHROMIUM_copy_compressed_texture GL_CHROMIUM_copy_texture GL_CHROMIUM_lose_context GL_CHROMIUM_sync_query GL_EXT_EGL_image_external_wrap_modes GL_EXT_blend_func_extended GL_EXT_blend_minmax GL_EXT_color_buffer_half_float GL_EXT_debug_marker GL_EXT_discard_framebuffer GL_EXT_disjoint_timer_query GL_EXT_draw_buffers GL_EXT_draw_elements_base_vertex GL_EXT_float_blend GL_EXT_frag_depth GL_EXT_instanced_arrays GL_EXT_map_buffer_range GL_EXT_multisampled_render_to_texture GL_EXT_occlusion_query_boolean GL_EXT_read_format_bgra GL_EXT_robustness GL_EXT_sRGB GL_EXT_shader_texture_lod GL_EXT_texture_compression_bptc GL_EXT_texture_compression_dxt1 GL_EXT_texture_compression_rgtc GL_EXT_texture_compression_s3tc_srgb GL_EXT_texture_filter_anisotropic GL_EXT_texture_format_BGRA8888 GL_EXT_texture_rg GL_EXT_texture_storage GL_EXT_unpack_subimage GL_KHR_debug GL_KHR_no_error GL_KHR_parallel_shader_compile GL_KHR_robust_buffer_access_behavior GL_NV_EGL_stream_consumer_external GL_NV_fence GL_NV_pack_subimage GL_NV_pixel_buffer_object GL_OES_EGL_image GL_OES_EGL_image_external GL_OES_compressed_EAC_R11_signed_texture GL_OES_compressed_EAC_R11_unsigned_texture GL_OES_compressed_EAC_RG11_signed_texture GL_OES_compressed_EAC_RG11_unsigned_texture GL_OES_compressed_ETC2_RGB8_texture GL_OES_compressed_ETC2_RGBA8_texture GL_OES_compressed_ETC2_punchthroughA_RGBA8_texture GL_OES_compressed_ETC2_punchthroughA_sRGB8_alpha_texture GL_OES_compressed_ETC2_sRGB8_alpha8_texture GL_OES_compressed_ETC2_sRGB8_texture GL_OES_depth24 GL_OES_depth32 GL_OES_draw_elements_base_vertex GL_OES_element_index_uint GL_OES_fbo_render_mipmap GL_OES_get_program_binary GL_OES_mapbuffer GL_OES_packed_depth_stencil GL_OES_rgb8_rgba8 GL_OES_standard_derivatives GL_OES_surfaceless_context GL_OES_texture_border_clamp GL_OES_texture_float GL_OES_texture_float_linear GL_OES_texture_half_float GL_OES_texture_half_float_linear GL_OES_texture_npot GL_OES_texture_stencil8 GL_OES_vertex_array_object GL_WEBGL_video_texture
     WebGL 1 Extensions: ANGLE_instanced_arrays EXT_blend_minmax EXT_color_buffer_half_float EXT_float_blend EXT_frag_depth EXT_shader_texture_lod EXT_sRGB EXT_texture_compression_bptc EXT_texture_compression_rgtc EXT_texture_filter_anisotropic MOZ_debug OES_element_index_uint OES_fbo_render_mipmap OES_standard_derivatives OES_texture_float OES_texture_float_linear OES_texture_half_float OES_texture_half_float_linear OES_vertex_array_object WEBGL_color_buffer_float WEBGL_compressed_texture_s3tc WEBGL_compressed_texture_s3tc_srgb WEBGL_debug_renderer_info WEBGL_debug_shaders WEBGL_depth_texture WEBGL_draw_buffers WEBGL_lose_context
     WebGL 2 Driver WSI Info: EGL_VENDOR: Google Inc. (adapter LUID: 0000000000011413)

EGL_VERSION: 1.5 (ANGLE 2.1.14226 git hash: 31a43497be09) EGL_EXTENSIONS: EGL_EXT_create_context_robustness EGL_ANGLE_d3d_share_handle_client_buffer EGL_ANGLE_d3d_texture_client_buffer EGL_ANGLE_surface_d3d_texture_2d_share_handle EGL_ANGLE_query_surface_pointer EGL_ANGLE_window_fixed_size EGL_ANGLE_keyed_mutex EGL_ANGLE_surface_orientation EGL_ANGLE_direct_composition EGL_NV_post_sub_buffer EGL_KHR_create_context EGL_EXT_device_query EGL_KHR_image EGL_KHR_image_base EGL_KHR_gl_texture_2D_image EGL_KHR_gl_texture_cubemap_image EGL_KHR_gl_renderbuffer_image EGL_KHR_get_all_proc_addresses EGL_KHR_stream EGL_KHR_stream_consumer_gltexture EGL_NV_stream_consumer_gltexture_yuv EGL_ANGLE_flexible_surface_compatibility EGL_ANGLE_stream_producer_d3d_texture EGL_ANGLE_create_context_webgl_compatibility EGL_CHROMIUM_create_context_bind_generates_resource EGL_CHROMIUM_sync_control EGL_EXT_pixel_format_float EGL_KHR_surfaceless_context EGL_ANGLE_display_texture_share_group EGL_ANGLE_display_semaphore_share_group EGL_ANGLE_create_context_client_arrays EGL_ANGLE_program_cache_control EGL_ANGLE_robust_resource_initialization EGL_ANGLE_create_context_extensions_enabled EGL_ANDROID_blob_cache EGL_ANDROID_recordable EGL_ANGLE_image_d3d11_texture EGL_ANGLE_create_context_backwards_compatible EGL_KHR_create_context_no_error EGL_KHR_reusable_sync EGL_EXTENSIONS(nullptr): EGL_EXT_client_extensions EGL_EXT_platform_base EGL_EXT_platform_device EGL_ANGLE_platform_angle EGL_ANGLE_platform_angle_d3d EGL_ANGLE_platform_angle_d3d11on12 EGL_ANGLE_device_creation EGL_ANGLE_device_creation_d3d11 EGL_ANGLE_experimental_present_path EGL_KHR_client_get_all_p

I see you have a Bitdefender toolbar. If BD is the main antivirus, add the TB profile folder as an exclusion. The profile folder location is shown in Help/More Troubleshooting, about:profiles. You might also want to disable the 'Advanced Threat Defense' option. The best way to test if BD or any startup app is affecting TB performance is to run Windows in safe mode.

https://www.bitdefender.com/consumer/support/answer/13427/

https://support.mozilla.org/en-US/questions/1275164

I added the TB exception which failed to help. I then turned off Bitdefender and Malwarebytes and there was no change.

I will check on safe mode later to see if something else might be conflicting.

Thanks for your prompt assistance.

The problem persists in safe mode.

Since there don't appear to be any obvious factors causing the issue, I suggest you go through this list to see if any apply to your setup: https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems

I checked all of the "Testing:Memory /Usage Problems" but none of those helped.

What next?

I would try a new profile with a single account. Test the new profile, and if works properly, data can be transferred from the old profile to the new. Create and manage profiles from Help/More Troubleshooting, about:profiles, or launch Profile Manager from a Run window (Winkey+R) by typing thunderbird.exe -p

That worked!

How do I transfer the information from the old profile to the new one?

Copy the local mail from the old profile to the new by copying mbox files from Mail/<popservername>, Mail/Local Folders, and sbd subdirectories, to Mail/Local Folders of the new profile. The mbox files are the large files with no extension, named after folders, e.g. Inbox. Copy abook.sqlite (Personal Address Book) and history.sqlite (Collected Addresses) from the old to the new. There are other files that can be transferred if necessary.

I copied the files you recommended as well as other recently modified data files from the old profile to the new one. That effectively restored my settings to the new problem-free profile.

Thank you for all of your help.

I copied the files you recommended as well as other recently modified data files from the old profile to the new one. That effectively restored my settings to the new problem-free profile. Thank you for all of your help.

Thanks for the update.

Unfortunate that the real cause was not identified. If any ideas should occur to you, please let us know.