There was a time when firefox was lean and fast, new pages came up near instantly and text you typed in even javascript heavy websites appeared just as quickly as you typed it...
Alas now a days my user experience has been so poor its lead me to find an alternative, now you may say, well... you're running on a nearly five year old laptop with "only" 8GB ram, but this is a laptop running a very light weight efficient distro (Void Linux) installed on a reasonably fast SSD, despite its age most applications are still responsive and more than usable even moderate use of Blender is more than realistic, and the kicker, run chromium with the same add ons and test again, both chromium and firefox but without add ons with identical results.
While Firefox chokes all 4 cores while loading a page (for a second or two!) and intermittently after a page seems to have loaded, in stark contrast with chromium I'm seeing full usage on one core and some usage of the other cores very briefly, and then no significant activity on my cpu meter, even when typing on some javascript heavy sites...
Quite whats going on I'm not entirely sure, could it be that Void's package maintainer be doing something wrong creating the template for building ? Well, possibly but I'd be surprised. Add on's have been ruled out thanks to safe mode performing just as badly, maybe its something else.
My biggest suspect would be bloat, take a look at the graph "LOC by language" notice since 2012 the trend is significantly upwards! from roughly 10 million lines of code! to a whopping roughly 35+ million lines of code.... staggering.... While chromium shows a similar trend ignoring comments and blank lines they're weighing in at 22+ million LOC so we're looking at, at least 10 million LOC less....
Sad to see, but it looks like firefox really needs to go on a diet!