Firefox

If you're forced to kill firefox, or it crashes, it will often leave a "hidden" lock file around. You will know this has happened when you open firefox and a window opens up asking if you want to to use "Default profile" or not.

find ~/.moz* -depth -name lock

This should find the lock file for you. If it has, then you can add | xargs rm to the end to remove it.

find ~/.moz* -depth -name lock | xargs rm

and now you can open firefox as per normal now.