https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding

https://www.basezap.com/cant-open-firefox-because-a-copy-of-firefox-is-already-open-mac/

https://apple.stackexchange.com/questions/196771/cant-open-firefox-because-a-copy-of-firefox-is-already-open

http://kb.mozillazine.org/Firefox.exe_always_open

It is also worth noting that this issue only occurs when Firefox is already running, and that clicking on the link when Firefox is closed will open the browser with the link in a new tab.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh).

After, try opening a link. Is the problem still there?

The problem is still there.

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia