Rated 4 out of 5 stars
MemChaser works well enough, but it's very obviously an alpha. The memory usage seems accurate. Although it doesn't match Task Manager (lol), it does match Process Explorer, and that's what matters.
It'd be handy if it implemented the functionality of this addon, saving me an addon:
They definitely need to fix the little red button. I don't want in in the bottom right. I either want to disable it or throw it in the menubar along with all the memory information. Resetting the position constantly is just downright annoying. I already have 20 other little icons down there from other addons that insist on throwing crap in the statusbar. (Or, sorry, it's an "Addonbar" now)
Well, MemChaser looks like it'll help.
You could disable extensions step by step to figure out which of those are causing the high memory usage. Otherwise check about:memory?verbose which lists compartments for websites and extensions. Probably it should be easier to figure out the causing extension. In any case it is worth filing a bug on Bugzilla. If you do so please cc :whimboo.