Occasionally you'll run into a site that has a long running script. Firefox gives you the option to stop the script or continue waiting (shouldn't each tab run scripts in its own thread anyway?).
Sometimes its quite tempting to click "Don't ask me again" and ignore the script dialog.
But if you ever want to get it back:
- Type "about:config" in the address bar
- Search for "dom.max_script_run_time"
- Reset the value