I don't know that it's for the exact same script each time, it may be.

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.

Script: chrome://messenger/content/folderPane.js:721

This is in association with Thunderbird I think.

This doesn't just happen after I've stepped away for a long time. It happens every time I click on anything. Things keep running that I'm clicking on.

Just before this started as a constant problem, I was looking at addons etc, and something in Thunderbird deactivated an old or expired JavaScript. I clicked on the update for that and after a while I found the best choice on the list and clicked download.

I don't know that anything happened with that and I moved away from addons etc area. The unresponsive script window started coming back all the time. Every time I clicked on something in Thunderbird and sometimes with Firefox. I turned off Firefox and restarted, but it didn't change. Shut down Firefox and computer restarted didn't change. The unresponsive script window is persistent now.