It really depends on the websites. Firefox has long been significantly faster than it was in the Firefox-1.x days, when it was the fastest browser. It's always been perfectly snappy and efficient for hackernews, craigslist, 10,000-line colored diffs in the cgit web frontend ... but popular web sites got many times slower. Chrome enabled web developers to do atrociously inefficient things and just not care.
Even today, Travis-CI really struggles in firefox, it's modern pleasing animations are just so atrociously coded. And remember when the Reason (facebook language) website was unveiled, it really killed firefox, because of some strange shadow effect applied to all letters or something ...