Ah, 3 words that always seem to show up together: firefox and memory and usage. I look forward to when I see that 4th word, "minimal", in there as well. Yes, I know original post is about an addin increasing memory usage, but I guess that doesn't surprise me anymore either, sadly, when thinking of firefox.
Even with the memory pain, I still (mostly) love FF.
It's much better than it used to be. For example, Firefox 15 made a class of very common leaks caused by add-ons impossible. Firefox 26 massively reduced the amount of memory used on image-heavy pages. And there have been many smaller improvements over the past three years, under the aegis of the MemShrink project (https://wiki.mozilla.org/Performance/MemShrink).
Even with the memory pain, I still (mostly) love FF.