Unending wait-for-load is evident by switching to an offending tab and checking the reload/cancel button. It will be stuck as an X (cancel) permanently until you click it and manually cancel.
What I'm asking for:
I'd like a configuration setting to impose a maximum load time limit across all tabs. Any tab that is loading for more than the specified limit will be automatically stopped (same as clicking the cancel button).
Due diligence (proof that I did my homework before asking):
I searched for anything that would do this and found a bunch of nonstarters:
Searching "firefox restrict maximum page load time" in Google turns up a bunch of performance tips aimed at faster page loads. Unfortunately that's of little use in a partially broken page that keeps loading forever.
This site (http://forums.mozillazine.org/viewtopic.php?f=38&t=2793647) said it can be done with an extension, although I don't see any extensions that do this.
Searching "firefox extension limit page load time" in Google only finds performance profiling and optimization, none of which are relevant.