Please provide the web link to this add-on.

https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox

Here is a link to the add-on. I'm able to install it on another PC, so I suspect the issue I'm having is specific to the environment on the PC where the install is failing.

https://addons.mozilla.org/en-US/firefox/addon/selenium-expert-selenium-ide/

Are you posting this from the problem computer?

Type about:support in the address bar and press Enter. Under the main banner, press the button; Copy Text To Clipboard.. Then in the reply box at the bottom of this page, do a right click in the box and select Paste. This will show us your system details. No Personal Information Is Collected.

That is a rather old extension: Released December 17, 2012 16.5 kB You can't expect much from such an old extension.

With what Firefox version did this extension work in case you have used it before?

I added the wrong link for the extension. I was downloading from: https://addons.mozilla.org/en-US/firefox/addon/selenium-ide/

This add on is still actively updated.

about:support output

Application Basics

Name: Firefox Version: 44.0.2 Build ID: 20160210153822 Update Channel: release User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:44.0) Gecko/20100101 Firefox/44.0 Multiprocess Windows: 0/1 (default: false) Safe Mode: false

Crash Reports for the Last 3 Days

All Crash Reports

Extensions

Name: Google™ Weather Version: 0.1.3 Enabled: true ID: jid0-lQyK6JstbAGdiq1fp28Cl@jetpack

Name: IDS_SS_NAME Version: IDS_SS_VERSION Enabled: false ID: {D19CA586-DD6C-4a0a-96F8-14644F340D60}

Graphics

Adapter Description: NVIDIA Quadro 2000M Adapter Description (GPU #2): Intel(R) HD Graphics Family Adapter Drivers: nvd3dumx,nvwgf2umx,nvwgf2umx nvd3dum,nvwgf2um,nvwgf2um Adapter Drivers (GPU #2): igdumd64 igd10umd64 igd10umd64 igdumdx32 igd10umd32 igd10umd32 Adapter RAM: 2048 Adapter RAM (GPU #2): Unknown Asynchronous Pan/Zoom: none Device ID: 0x0dda Device ID (GPU #2): 0x0126 Direct2D Enabled: true DirectWrite Enabled: true (6.2.9200.17568) Driver Date: 6-5-2011 Driver Date (GPU #2): 6-10-2011 Driver Version: 8.17.12.6883 Driver Version (GPU #2): 8.15.10.2418 GPU #2 Active: false GPU Accelerated Windows: 1/1 Direct3D 11 (OMTC) Subsys ID: 14a31028 Subsys ID (GPU #2): 04a31028 Supports Hardware H264 Decoding: Yes Vendor ID: 0x10de Vendor ID (GPU #2): 0x8086 WebGL Renderer: Google Inc. -- ANGLE (NVIDIA Quadro 2000M Direct3D11 vs_5_0 ps_5_0) windowLayerManagerRemote: true AzureCanvasBackend: direct2d 1.1 AzureContentBackend: direct2d 1.1 AzureFallbackCanvasBackend: cairo AzureSkiaAccelerated: 0

Important Modified Preferences

browser.cache.disk.capacity: 358400 browser.cache.disk.filesystem_reported: 1 browser.cache.disk.smart_size.first_run: false browser.cache.frecency_experiment: 4 browser.download.importedFromSqlite: true browser.places.smartBookmarksVersion: 7 browser.sessionstore.upgradeBackup.latestBuildID: 20160210153822 browser.startup.homepage_override.buildID: 20160210153822 browser.startup.homepage_override.mstone: 44.0.2 dom.apps.reset-permissions: true dom.mozApps.used: true extensions.lastAppVersion: 44.0.2 gfx.crash-guard.d3d11layers.appVersion: 44.0.2 gfx.crash-guard.d3d11layers.deviceID: 0x0dda gfx.crash-guard.d3d11layers.driverVersion: 8.17.12.6883 gfx.crash-guard.d3d11layers.feature-d2d: true gfx.crash-guard.d3d11layers.feature-d3d11: true gfx.crash-guard.status.d3d11layers: 2 gfx.crash-guard.status.d3d9video: 2 gfx.direct3d.last_used_feature_level_idx: 0 media.gmp-eme-adobe.abi: x86-msvc-x64 media.gmp-eme-adobe.lastUpdate: 1455304393 media.gmp-eme-adobe.version: 15 media.gmp-gmpopenh264.abi: x86-msvc-x64 media.gmp-gmpopenh264.lastUpdate: 1455304394 media.gmp-gmpopenh264.version: 1.5.3 media.gmp-manager.buildID: 20160210153822 media.gmp-manager.lastCheck: 1455304388 media.hardware-video-decoding.failed: false network.cookie.prefsMigrated: true network.predictor.cleaned-up: true places.database.lastMaintenance: 1455311488 places.history.expiration.transient_current_max_pages: 104858 plugin.disable_full_page_plugin_for_types: application/pdf plugin.importedState: true privacy.sanitize.migrateClearSavedPwdsOnExit: true storage.vacuum.last.index: 1 storage.vacuum.last.places.sqlite: 1455311488

Important Locked Preferences

JavaScript

Incremental GC: true

Accessibility

Activated: false Prevent Accessibility: 0

Library Versions

NSPR Expected minimum version: 4.11 Version in use: 4.11

NSS Expected minimum version: 3.21 Basic ECC Version in use: 3.21 Basic ECC

NSSSMIME Expected minimum version: 3.21 Basic ECC Version in use: 3.21 Basic ECC

NSSSSL Expected minimum version: 3.21 Basic ECC Version in use: 3.21 Basic ECC

NSSUTIL Expected minimum version: 3.21 Version in use: 3.21

Experimental Features

bmk11 said

This add-on is still actively updated.

That one has not been signed. It might be the problem.

This will disable add-on signature enforcement; Enter about:config into your Firefox address bar Click the I'll be careful button Find xpinstall.signatures.required in the list Right click the preference Select Toggle until the value is false https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox

https://wiki.mozilla.org/Firefox/AddOns/Status/Updates#Exec_Summary “Currently targeting Signing with no pref to turn off in Firefox 47”