Start Firefox using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

Did as you asked. In Safe Mode (definitely not refresh mode), the problem goes away. Right-click works perfectly even inside folders. Just as it used to. I did not recently change any settings, added extensions, or changed themes. Not for weeks, possibly months. Any ideas?

Sorry for the late reply. Been in Hospital.

https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode

Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes), any userChrome.css/userContent.css files, and some other advanced features to help you assess whether these are causing the problem. If the problem goes away, that tells us one thing. If it does not, that tells us something else.