This happens because I usually leave Firefox running on my Desktop, when I go to the lecture hall. This source explains the problem:
Some applications (e.g. firefox, eclipse and more) use lock files to prevent users from opening two instances of the same application. A lock file is usually a simple file named "lock" or ".lock".On a linux box, the solution is relatively straightforward:
The application creates the lock file when it starts, and deletes it when it exits. When it crashes, it might forget to clean up the lock files, and when it is re-opened, and finds the lock files, it thinks there is already an instance running, and as a result, refuses to open.
1. Go to the .mozilla/firefox, ./thunderbird, or ./mozilla-thunderbird directory
2. There is often a directory "X" which ends with ".default". cd into this directory.
3. Remove the "lock" file.
4. Of course, when you delete the lock, the session that the lock is protecting will probably crash.