Possibly more than 1 left over Extension you have tried out.

Please Test in : SAFE MODE

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

In Firefox Safe mode these changes are effective:

• all extensions are disabled (about:addons)
• default theme is used (no persona)
• userChrome.css and userContent.css are ignored (chrome folder)
• default toolbar layout is used (file: localstore-safe.rdf)
• Javascript JIT compilers are disabled (prefs: javascript.options.*jit)
• hardware acceleration is disabled (Options > Performance > Uncheck to view)
• plugins are not affected
• preferences are not affected

TEST''''is issue still there ?

If so please : Refresh but Note that this does more than uninstalling and re-installing does. Normal uninstalling does not remove some preferences which may have become corrupt. Please Refresh but do this 1st:

• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer
• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

then

• https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

Thanks for the info. This is sporadic behavior which is almost always corrected with a restart. So restarting in safe mode probably wouldn't tell me much?

Incidentally, this behavior also occurs sporadically in the Pale Moon browser (in case that sheds light on anything)

H, not really as Pale Moon has changed. Note : Firefox update Sept 5th, PaleMoon.org updated Pale Moon to fix bugs, then updated it again to fix more bugs. Users should update. Caution: naïve browser snif­fers may identify Pale Moon as Firefox 52.9

Suggest run in Safe Mode and try and see though you can try this : uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files , C:\Program Files(x86) & C:\ProgramData Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do :

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 61.0.2 with a Full Version Installer

• https://www.mozilla.org/firefox/all/

If still a issue : Profile : Make a new one and test ...:

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

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