I use Firefox almost all the time (on 5 machines) and before that when it was Mozilla and/or Netscape as my primary browser. As for resource usage, had a look and Chrome (use it only to run stuff that won't run on my NoScripted/adblocked/Flash-blocked and other junk blocked FF) seems to be hogging as much if not more memory and I have many more extensions running on FF (which updates in the background - I only know there's an upgrade when I cold boot which is every morning). I generally use IE or Edge for cash back buying as the others have ad blockers. I find Edge slow and doggy.
FWIW, 2 Firefox windows, one running 14 tabs is using just a shade under 600 MB (on a 8 GB i5-3230M CPU laptop). Chrome running Facebook (with FB Purity to block just about everything else) on Chrome is running 171 MB.
Just for fun, I closed one FF window and opened all the tabs I had on the FF window on the Chrome window (and closed the FB tab). FF is now using 545+ MB, Chrome 880+ MB. Cut back to 6 tabs (same pages) on each and it's ~477MB on FF, 61MB on Chrome.