I have about the same problem... Tried the same steps as that user to no avail. Also tried the about:memory Minimize memory usage button, didn't do anything. (I have been checking the memory usage via About:memory total, and task manager.)
Although recently Adobe flash has also been rising with it! a few moments ago, both rose to 1.5+Gb ram each and firefox hung and gave 2 non responsive script errors. Almost crashed. Sometimes it does. Last real crash was just yesterday. ALSO they both got up to 10 percent CPU each.
Before I updated adobe I guess??? it was just firefox, rising and rising, from opening and CLOSING new tabs. (now both do) anywhere from 20-30 tabs max with opening and closing frequently, can cause this and ff can become an unusable browser within 30 mins or so to several hours, depends.
right now it is doing not too good but better than what I reported above, don't worry it will grow, unless I restart ff... then it will grow again later..
I saw that it could be a memory leak on some sites, and i have some garbage collection logs.. if it matters.. don't know if they were supposed to be taken at a certain time or what though.
Pleaaaase I appreciate any help, thank you!