Nothing new, FF has always had an history of being a memory hog with plenty of memory leaks.
Close browser and restart when that happens.
However, in spite of this, FF IS my preferred go to browser because I can add a few addons like AdBlockPlus NoScript which makes it a much more secure browser with a lot less chance of driveby malware taking a stab at my PCs.
Win7 Pro? Yeah, like myself running some PCs with XP, you NEED to run a browser with as many types of lockdowns as possible since MS has discontinued all support for non commercial customers.
Chrome and IE, no so much for security and in a lot of cases the last version of IE 11 which can be run on Win7 is not accepted by many websites like ADP and banks. Ironically meaning even Win8, Win8.1 and Win10 folks using IE11 have the same issue of some websites rejecting IE11.