Well, as one piece of anecdotal evidence: I only use one browser extension (Vimium on Chrome, VimFx on Firefox) and still find Firefox significantly faster than Chrome.

Also, I like using the same browser on my Android tablet as on my laptop and websites that use MathJax render incorrectly on Android Chrome (these sites render perfectly well on Android Firefox and on both desktop Firefox and desktop Chrome). So I'd probably use Firefox even if it were a little slower than Chrome.