This message is displayed by Firefox whenever some javascript is taking a long time to run. It can happen with a TiddlyWiki file if you have a lot of tiddlers or are using some plugins that do some extra work that takes a long time. You can get rid of the messages by making Firefox's maximum script run time a little longer. Here's how it's done:
- Type about:config in the address bar and press enter.
- Scroll down until you see dom.max_script_run_time
- Double click it and type in 20 seconds (the default is 5 seconds)