Script busy or stopped responding
From MozillaZine Knowledge Base
This warning message:
A script on this page may be busy, or it may have stopped responding. You can stop the script now, or you can continue to see if the script will complete
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.
Increasing the time limit
If you often use scripts that take a long time, then you can increase the time limit.
For scripts stored in messages, set the preference: dom.max_script_run_time
For scripts in Thunderbird itself, or in extensions, set the preference: dom.max_chrome_script_run_time
Specify the time limit in seconds. You can specify 0 to allow scripts to run for a very, very long time (effectively forever).
Note that if you specify a long time, then any script that has a bug might run for that length of time before you can stop it cleanly. This means that long timeouts, and especially zero, are not always a good idea.
Finding similar problems in Thunderbird or Mozilla Suite or SeaMonkey
Finding others with similar problems may find something that helps.