The popular Firefox web Browser isn't written for concurrency within a user profile. That means that you can have only one open instance of the program per user. If your account is not local to your machine, you must close all previous instances before trying to start a new browsing session. On occassion, Firefox will not start, because previous sessions were not closed properly (because of a crash or network interruption.)
##On Mac OS X
To fix this issue on OS X, please log out, and log back in. You may also need to delete your parentlock file, located at:
Before attempting the following steps, please make sure that you are currently logged into only one computer. Often times Linux users have multiple Firefox session running, each session use the same lock file which causes Firefox to get confused.
Here are some steps to use Firefox, if it becomes locked by mistake:
rm -f ~/.mozilla/firefox/*.default/.parentlock
rm -f ~/.mozilla/firefox/*.default/lock
After you do so, try to run firefox one more time.
In the worst case scenario, you could do this:
mv /.mozilla/firefox /.mozilla/firefox_old
(Note: this will completely reset Firefox.)
Then start your browser, which will create a new profile. After you do this, you may wish to restore your bookmarks. You can do this via the Firefox menu path
Bookmarks/Organize Bookmarks/File/Import (your old bookmarks would be in