I'm also using ubuntu 13.04 wich is not quiet well de-bugged and sometimes crashes leaving my firefox profile in dead memory. So when I restart the computer and run firefox it says : "Your Firefox profile cannot be loaded. It may be missing or inaccessible."
Just go to
/home/uname/.mozilla/firefox
and rename your backed-up profile folder abcdefg1.default.backu
p (you will find its path in the profiles.ini
file) to abcdefg1.default
Firefox will start again with an old profile but it works !