Hi,
What happened before this issue occurred? Did you delete the folder containing your profile? If you manually deleted it, you may need to restore the folder to its original location. Otherwise, you can follow the steps in the official article to resolve the issue.
https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible
Disclaimer: This is a non-Microsoft site. The information seems reliable, but be careful with ads. Research any product before downloading.