I upload photos to Flickr and Zooomr pretty regularly. Both sites use a script for the upload, and that script understandably runs for a long time while waiting for the upload to finish, especially when uploading multiple photos.
Firefox is impatient when it comes to scripts, and has a built-in timeout designed to protect you from poorly written scripts that may run away with your CPU. Since the scripts that Flickr and Zooomr use spend a lot of time waiting, Firefox thinks that it has timed out. As a result, it pops up an error message for the "Unresponsive script" and asks you if you want to terminate it. If you choose to continue, it will ask again every five seconds until the script has completed. Very annoying.
You can change how long Firefox will wait before it thinks that a script has timed out (from Rob Cottingham):
1. Type about:config in Firefox's address bar.I don't know about you, but my uploads generally take longer than 20 seconds, so I changed my timeout to 120 (two minutes). It's probably wise not to set it inordinately high, since the timeout may come in handy for what it was intended for: poorly written scripts. If you set that value too high, you'll probably end up having to terminate Firefox before it gets around to stopping the offending script.
2. Filter down to the value for dom.max_script_run_time.
3. Change the value to something higher than the default (which is 5.) I set mine to 20.