Firefox kept hanging on some pages running scripts that take to long to run.
An attempt to fix the problem led me to this page:
http://kb.mozillazine.org/Dom.max_chrome_script_run_time
JavaScript that takes a long time to run can make the browser unresponsive. As such, a check is built in to allow long-running scripts to be aborted. That check is now able to differentiate between scripts running in web pages (content) and scripts running in the application’s chrome. This preference determines how long scripts running from chrome are allowed to run before the user can choose to abort the script.
What is an applications chrome in relation to javascript? Searching brings up info about Google chrome browser.