What security software do you have?

Make sure that other software isn't protecting the profiles.ini file against changes.

You need to locate the profiles.ini file and delete this file or use the Profile Manager to create a new profile.

See:

• https://support.mozilla.org/kb/how-run-firefox-when-profile-missing-inaccessible

This issue can be caused by a problem with the profiles.ini file or with the permissions (owner) of involved folders.

You can use one of these to make Firefox create a new default profile or recover an existing profile:

• Delete the profiles.ini file to make Firefox create a new (default) profile
• Use the Profile Manager to create a new profile
  If you still have an existing profile then click "Choose Folder" in the Profile Manager and browse to the location of a lost profile to recover this profile
  https://support.mozilla.org/kb/Managing+profiles

You should find the profiles.ini file in this hidden location:

• Windows: %AppData%\Mozilla\Firefox\
  *C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\
• Mac: ~/Library/Application Support/Firefox/
• Linux: ~/.mozilla/firefox/

If clearing the profiles.ini file isn't working then also check the location of the cache files.

• Windows: *C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\<profile>\
• Mac: ~/Library/Caches/Firefox/Profiles/
• Linux:~/.cache/mozilla/firefox/

You can also try to rename/delete the Mozilla\Firefox\ folder in both above mentioned locations (main, cache).

• http://kb.mozillazine.org/Profile_Manager
• https://support.mozilla.org/kb/Managing+profiles
• http://kb.mozillazine.org/Profile_folder_-_Firefox

I'd already done everything you'd mentioned above before I posted. None of it fixed the problem. But I did eventually fix it on my own, and here is how:

The umpteenth time I'd gone into the profile manager to delete the profile yet again, I hovered over word "default" and saw the whole path to the file (which I had also visited more than once). However, I had the window open to the AppData\Local\Mozilla\Firefox\Profiles\ and noticed that the file it was looking for did not match the name of the file that was there. I deleted the profile, made a new one, and tried again. Same thing happened.

What I found interesting about this was that "default" was trying to open the SAME nonexistent profile -- it was stuck on the idea that the default file was supposed to be named "blahblahblah123" (or whatever it was), no matter what was actually in the folder or how many times I tried deleting/make a new profile.

So I wrote down what the program seemed determined the default file was supposed to be named, went into the Mozilla\Firefox\Profile\ and changed the remade profile name to match it that name.

Fixed. Haven't had the problem since.

Also fixed my Thunderbird file that was doing the exact same thing -- it was stuck on the idea it should be named a certain thing no matter how many times I deleted and remade the profile.

I have to wonder, since I synced to my old machine I am retiring, if those are the default profile names from that computer, and my new computer was trying to recycle those names.

Did you try to rename or delete the profiles.ini file?

Issues like this are almost always caused by other (security) software that is protecting the profiles.ini file against changes, but doesn't notify you. You should be able to use the Profile Manager to delete a profile and create a new profile. Renaming a profile on the hard drive is not recommended because some files may contain a reference (link) to the original profile that now no longer exists.