I uninstalled and reinstalled and tried to restore my old profile, but I can't open Firefox, it says another copy is already open. How do I fix that?

Question

I followed the steps to backup my profile and saved the file on my desktop. I uninstalled Firefox and reinstalled it. I tried to restore my profile by copying it to the "new profile" but it didn't work, my bookmarks etc did not come back. When I looked in the new profile my old one was there as a folder so I moved it out to the Profiles section. I now had 2 profiles. I deleted the new profile using the Profile manager, thinking the old one that was left would just take over. I then tried to open Firefox but it says
Close Firefox
A copy of Firefox is already open. Only one copy of Firefox can be open at a time.
Where is this other copy? How do I close it?
Thank you.

Answer 1

Did you check the File > Work Offline setting?
Press F10 or press and hold the Alt key down to bring up the "Menu Bar" temporarily.
A possible cause is security software (firewall) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.
Remove all rules for Firefox from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process.
See:
* [[Server not found]]
* [[Firewalls]]