The real problem is that once all that memory is allocated, it never gets let go. It' perfectly fine to say that your copy of Firefox doesn't exceed 300 MB of memory usage in most cases, but if it does, only a restart can fix it. That may not be a "leak" in the traditional sense, which is why the geeks are upset about the meme, but the end result is the same. The problem is very much real, and has been for, oh... about 8 years!
AdBlock is greediest memory hog. After enabling that one extension, 10 minutes of browsing bloats memory usage to beyond 1GB and the browser slows down BIG time... to the point where it's practically unusable.
The infamous "pausing" problems are also still present. They occur only when Firefox has allocated a huge amount of memory, and are probably related to either garbage collection or session state saves. Firefox really needs some sanity checks, or at least some tweaks so massive memory usage doesn't kill performance.