For my sins, I periodically use a number of Javascript-heavy websites (some of them are sites where the Javascript is core to the experience). I have similar Javascript enabled environments in both Chrome (in incognito mode) and Firefox, with relatively similar sets of extensions (Firefox version and Chrome version). My consistent experience over the past while is that Chrome is clearly faster on these sites than Firefox, and in particular Firefox often feels actively laggy.

(Years ago I sort of had the same experience on Flickr, but I believe that more or less went away later for a while. It returned a couple of years ago, and I quietly switched from using Firefox on Flickr to mostly using Chrome.)

This result sort of surprises and depresses me (partly because using Chrome has its pains). My understanding is that in theory Firefox and Chrome are usually relatively neck and neck as far as performance goes, with Firefox at least competitive, and that especially on common sites Firefox should not be laggy. There are a number of things that could be causing this for me and not for other people, especially general users. For a start I'm on Linux and using Fedora's build of Firefox instead of the Mozilla build, while I think most performance comparisons are made on Windows or MacOS and use the official Mozilla builds.

(I'm also using a relatively odd Linux environment with relatively modest OpenGL and compositing support, which might hurt Firefox more than Chrome.)

Beyond that, possibly my core Firefox extensions are slowing down Firefox more than I expect. But if so, well, they're my core extensions for a good reason (and the obvious suspect of NoScript is not entirely the cause, since some of my Firefox usage is without it). What matters to me is the performance of the browsers I've created from Firefox and Chrome, not the performance of the stock versions in some platonic ideal state that I will never use them in. Given that I have one decently performing browser, that's what I'll wind up using for Javascript-heavy sites even if it's not Firefox.

(And I'm using some extensions in Chrome's incognito mode that I would expect to be sort of heavyweight, like uBlock Origin and a mouse gestures extension.)

PS: I care about this partly because I dislike some things Google does with Chrome and partly because I care about Firefox being competitive and good in general. The overall web ecology benefits when we have a real choice in browsers, and part of having a real choice is good performance.

(I also think that Mozilla fundamentally cares more about Linux for Firefox than Google does for Chrome. As a non-Windows, non-Mac user, I remember the days when I was a second class citizen on the web and I would rather like to not go too far back to them.)