There used to be a big difference (in JS engine speed) but it's small recently. Where there is still a difference is in the UI code. Not necessarily operations being slower in Ff, but ordered in such a way to appear slower.

For example, when you press ctrl+n, chrome instantly creates a new window, then spends the next second or so (depending how old your computer is) painting the UI. On Firefox though it seems like nothing is happening for an entire second, and then the window emerges fully painted. So even tho the total time is about the same, FF feels much less responsive due to this design choice.

I also noticed Firefox spent a much longer time (a fraction of a second, but perceptible, and multiply that by thousands of clicks a day) before it would even (apparently, again -- not sure whats going on inside) begin loading a page, which was frustrating.

I verified this was the case in the web inspector's network performance tab (there was an mysterious delay before the first network request).

I've been experiencing delays too but only on the first request. This was due to uBlock Origin initializing. On mobile, this load time adds up really fast as it takes around 5 to 10 seconds for uBlock to load. In the mean time, requests are configured to be delayed. The aggressive Android memory cleaner doesn't help as anytime you close your browser, it reloads again.

All this to say, you might have an add-on delaying network requests to match them against a blacklist, save metadata or whatever the add-on is supposed to be doing.

> Firefox uses their own engine called Spider Monkey. I don’t know of anything outside of Mozilla which uses it.

Some non-Mozilla projects do embed SpiderMonkey, including Bloomberg (since 2001!), MongoDB, CouchDB, and 0 A.D.

https://discourse.mozilla.org/t/survey-where-are-you-embeddi...