Firefox extension Leak Monitor keeps an eye out for memory leaks in Firefox, alerting you each time an extension has sprung a leak.
Although Leak Monitor is geared toward developers, it's not too difficult to wade through the leak alert and find out what extensions are causing the most problems; for example, I was surprised to find that my Adblock extension seemed to have a pretty steady memory leak. You probably won't want to leave the Leak Monitor running all the time, but it's a good way to hunt down a leaky extension if, like me, you find your Firefox memory usage has somehow crept up to 500MB in the course of a day and you suspect that an extension or two might be at fault. Thanks Nemo! — Adam Pash