"[This] means that some internal script ran out of time. This can happen when a script has a lot of work to do, or it can happen when the script has a bug.
If you suspect that the script has a bug, or if you do not know what script is running, then press the "Stop script" button to stop the script.
If you know that some lengthy operation is in progress, then press the Continue button to allow the script to continue working. You might have to do this repeatedly to allow the script to finish.
Examples of operations that can be lengthy are: sending a message using a very large mailing list, fetching new mail using the webmail extension, sending personalized messages using the Mail Tweak extension."
Source and further information:
http://kb.mozillazine.org/Script_busy_or_stopped_responding