To be absolutely clear - the icon bounces up and down, but no window is present and no screen comes up. Nothing happens until I force-quit the application, and then it's just macOS telling me it'll send a report to Apple.

Sorry Daniel that your thread slipped thru & got missed!

Is this still happening? If I were you, I'd backup the entire Firefox profile directory then uninstall & reinstall Firefox because something must have definitely corrupted it or attached itself to Firefox to crash it instantly like in that other thread you found.

Locate your Firefox profile & back it up: https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data#w_finding-your-profile-without-opening-firefox Should be in ~/Library/Application Support/Firefox/Profiles/

A more detailed look at what files are in your Firefox profile: https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile#w_your-important-data-and-their-files

Troubleshoot Mode Is using the Option key + mouseclick really not opening Firefox at all? It should force Firefox to load in Troubleshooting Mode where it disables many things including addons. To start Firefox in Troubleshoot Mode, hold down the Option key & click the Firefox icon at the same time.

More info about Troubleshoot Mode: https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-troubleshoot-mode

Can you also check Activity Monitor to see if any Firefox process appears while you try to open it?

Hi NoahSUMO, thanks for the reply. Had assumed this was abandoned. When I `rm -rf`d the Firefox directory in Application Support this seemed to do the trick, as I had attempted to uninstall and reinstall Firefox already. When I did this I had no issues installing Firefox clean. I guess it tidies up my browser I suppose. Thanks for the help.

You're welcome Daniel. Really glad that rm -rf method worked. Something must've really been stuck in that Firefox directory that needed to be updated back to its original self. Sounds like one of the critical files got corrupted somehow. Maybe during an incomplete update. But you would think uninstalling & reinstalling Firefox the normal way would've been enough.

Glad it works now & sorry for leaving you hanging for so long! Happens sometimes if we get a flood of questions or a flood of spam.