Hi guthertzgroup, I'm sorry to hear about this problem. Usually this indicates that Firefox checked a configuration file for your profile folder name and path, and the folder is not there. So one possible avenue of explorer is to try to recover it from a backup. But until then, it might be useful to have a new profile to browser in. Please see the following article for more information: [[How to run Firefox when your profile is missing or inaccessible]] In the right column is an OS selector. If that doesn't switch to MacOS automatically, please change it manually because the steps are different on Mac than Windows. Can you get a new profile up and running?

See also: *https://appletoolbox.com/2016/07/fix-corrupt-user-accounts-macos/