Comment from a newbie as a looooong time Firefox user.
My memory leak issues with Firefox seem to be related to specific web pages/sites.
AND, it does appear related to the amount of information on the web site, that ends up downloaded on ONE page.
My biggest problem is with "Aliexpress.com". Some of the pages will cause memory use in Firefox to jump from say 600 meg, to well over 2 GB of memory, in about 10 minutes. Or less. You can watch it steadily climb in Task Manager. In some cases, it climbs fast enough that it finally freezes the computer and I have to do a forced reboot. (If I catch it quick enough, I can do an "end task" on Firefox within Task Manager and stop it from killing the whole machine.
Also, when it does this, I can close every single window except for just a basic google search window, and the memory is NOT released. It's as if once the memory is used like this, it's stuck until you close Firefox completely, then restart it.
I'll try and bookmark the pages that it happens on and post a couple. It does not do it on every page. Only some of them.