One thing you'll often see is that "new" browsers - i.e. with no browser history, extensions, etc. - perform better than one you've been using for a while. Thus, when people switch browsers, the new one tends to feel faster, regardless of what you're switching to.

Possibly, Firefox feels just as fast after a refresh. Alternatively, a fresh profile using the new profile manager might do the same.

On the other hand, it could also be that that specific site has just been tested and optimised in Chrome, with Firefox mostly ignored because too few people use it :(

I've been on Android since 1.5 and the "snappy" claims remind me a lot of the claims every new version there too. They're likely still going, though I haven't been active on XDA for a long time to know.

Is it actually objectively faster? Or is it a perception thing based on the way animations and screen changes are handled?

yup. as someone who used firefox in the "wasn't yet called firefox"-day, switched to chrome when it came available on linux, and then had to switch back to firefox a year ago: chrome feels just so. much. snappier.

It's hard to describe, but everything just feels... slightly off

You can try cromite, I think it has a version for PCs