Use the Profile Manager to create a new profile.

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

If the Profile Manager still ists profiles then it is likely that those profiles no longer exist on your hard drive.

• http://kb.mozillazine.org/Profile_folder_-_Firefox

You can also delete the profiles.ini file to force Firefox to create a new profiles.ini file and a new default profile.

• http://kb.mozillazine.org/profiles.ini_file

Firefox is not broken in Ubuntu, as cor-el stated, you just need to create a new profile. If you would like to, you can transfer data from the old profile to the new one.

• Recover important data from an old profile

Note that this may break your new profile if you transfer corrupt stuff.

The profile manager doesn't run either. It issues the same error: "Your firefox profile cannot be loaded. It may be missing or unaccessible".

Something definetly IS broken if a profile gets corrupted by merely updating firefox. (especially considering the profile was almost untouched: I had only run firefox a couple of times)

Additionally, the profile manager is almost useless, if it becomes unusable just because one profile is corrupted (even if there was a legitimate reason for the profile being corrupted in the first place).

Ouch!!! Now I HAVE deleted prifiles.ini (fortunately there was nothing important in my profile, as it was a fresh new unused profile, so wiping it out would not be a problem in this case), but it STILL doesn't work.

Hm, this intriguing error message that shows up when trying to run firefox -profilemanager from the command line, may shed some light on this:

Error: Access was denied while trying to open files in your profile directory.

This looked like https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1180227

but that is supposed to be fixed, and the apparmore rule described in that report are right there. Also, the apparmor profile for firefox seems to be disabled by default, so that should be irrelevant.

aa-enforce and aa-disable didn't change a thing.

Now I've realized my .cache/mozilla folder belongs to root rather than to me (of course I hadn't touched it). By chowning it to my user, it seemed to fix the issue at first. I could run the profile manager and I'm almost sure I ran firefox once (I closed it so quickly I can't tell for sure).

But now, firefox won't run anymore. Now it doesn't ssue any error message at all, unless I run it from command line, in which case there's this message:

(process:9539): GLib-CRITICAL **: g_slice_set_config: assertion `sys_page_size == 0' failed

profile manager now works.

deleting ~/.cache/mozilla finally fixed it.

you can solve it so: 1) in Terminal write firefox -P 2) then press the botton delete profile 3) press create new profile you can choose the name you want ,or let as default user

finally press Finish