Firefox developers have been claiming that plugins were to blame for a long time (and for some FF versions they may have even been right, judging from TFA). However, there was no way to do something about it, or even just find out which plugin was the problem. TFA finally gets this:
Although these leaks are not Mozilla’s fault, they are Mozilla’s problem. Many Firefox users have add-ons installed -- some people have 20 or 30 or more -- and Firefox gets blamed for the sins of its add-ons.
Now they are going to improve reviews and make it possible to mark add-ons as memory-hogs / -leakers.