If you're getting the "Warning: Unresponsive Script" with Firefox on a specific web site, more often than not it's a JavaScript issue. There may be an error with the JavaScript configuration, or it may be by design. I'll leave it the mods and admin to sort that issue out.
Before dealing with JavaScript, check the version of Firefox you're running. If it's not the current (23.0) update it and check to see if the warning still persists, or is gone. Second, Webroot Spy Sweeper is sometimes the culprit. If you're running WSS, disable it and check.
If the above fails and the problem still persists then it's time to deal with JavaScript. There are ways of changing the default time and making it run longer before the warning message appears, but that won't solve the root problem. You can also block JavaScript altogether but that may not be desirable as a few sites might not function properly. And then, there's a very simple and harmless Firefox add-on known as YesScript that will block JavaScript on a specific site. I won't post the instructions here as that may be against forum policy so you need to check it out yourself. Just check Firefox add-ons for the download and installation procedures.
I installed it because of issues I had on another site and it worked perfectly. I've not had a need to install t on SMF because I don't have the problem mentioned here.
Good luck.