I am having a similar ongoing issue with Firefox refusing to open, and none of the fixes I've seen in this forum have worked. When clicking the icon it bounces once and then disappears from the dock. Honestly, this started happening about August 2018, and in my troubleshooting decided to give the Developer edition a try, and for some reason that one worked so I stuck with it. Shortly thereafter (a week or two) whenever I closed down the Mac, Firefox Dev wouldn't re-open. Deleting and reinstalling would work, but I'd have to re-set all my settings, re-activate the theme, etc., and all the browsing history would be lost. So, since then I would rarely shut it down. A few days ago I had to shut the Mac down completely, but this time Firefox Dev wouldn't open after re-installing. Oddly, I tried regular Firefox again as a solution, and it opened this time. I tested it out by shutting the Mac down completely, then restarting, and it retained all of the settings! I was really glad to once again be able to shut down my laptop at the end of the day without the headache of having to reinstall and re-set everything. Then this morning, boom - back to the same nonsense, except now it won't open at all. Neither the regular FF or the Developer edition will work. I've done clean uninstalls to see if that was the problem, as in: removed them from the Applications folder, removed them from the Downloads folder, removed the /Firefox folder from user/Library/Application Support/, and emptying the Trash. I saw in one Support post to delete the fbplugin file, 'as that had been causing Firefox to crash'. The closest I found to an 'fbplugin' file was a bundle-config.1 file, which contained 'fbplugin' throughout the file code numerous times. I renamed it temporarily, but didn't delete it. That didn't help the non-opening problem either. I also tried installing Firefox v 71, same result as the others. Keep in mind after attempting all of these solutions of deleting, reinstalling, changing file names, etc., I did a complete shutdown and restart. Also, I have not downloaded, updated, or changed anything since last week's phenomenon of regular Firefox beginning to work again. So I'm completely stumped and would appreciate any help. I'm currently running macOS Catalina 10.15.3. TIA.