You might be interested in the javascript debugger: https://developer.mozilla.org/en-US/docs/Tools/Debugger Under the "How to" section it decribes how to pause and control execution.

EDIT: Since you mentioned addons as an option, I myself use the NoScript security suite: https://addons.mozilla.org/en-US/firefox/addon/noscript/ . You can not only enable / disable scripts on a page quickly, but also scripts from specific sources.

Thanks, found it: Ctrl-Shift-Z (in firefox) then F8 to halt scripts Ctrl-Shift-S (in Waterfox) then F8 to halt scripts

Firefox takes a horrible turn and makes the web page unaccessible, why? Thankfully waterfox doesn't do that.