You could try this please : Go the 3 Bar Menu then Options --> General --> Performance and untick everything. change the recommended size lower then see how it runs. Note: 1 = No Multiprocessor = slow again. Try 2 Restart Firefox after making these changes please.

Note : Acceleration is for your Video Card, monitor to see if need to turn back on. https://support.mozilla.org/en-US/kb/performance-settings

• Please leave the check mark in Prevent Accessibility Services.

Please let us know if this solved your issue or if need further assistance

There is nothing against using only one content process versus multiple content processes in case of performance issues. Only if you completely disable multi-process then there isn't a separate content process and a crash in a tab will take down Firefox or the user interface will respond slower.