I'm on a Mac, a 2015 MacBook Air and a late-2012 Mac Mini, and don't have any perf problems with Firefox, using the standard, public builds (so not the dev ones). This is with an around 10-15 tabs open, with sync, and 10 add-ons of various stripes.

Then again, I don't do heavy web development or have a tab open with e-mail all day (e-mail is in Thunderbird) so maybe that makes a difference?