Lots of little things that might not matter much individually, but still add up significantly when taken together:

- tabs can't be reordered any more

- bookmarks/history/recently closed tabs/top sites... are more cumbersome to access

- no "recently closed tabs" for quite a few months after the initial release

- bookmarks always force-open a new tab

- search suggestions needlessly take up more space (apparently by design, because these days information density is baaaaad), so you can't see local history/bookmark results without closing the keyboard or scrolling down

- using a search engine other than the default one requires two extra click at least, whereas previously they were directly accessible at the bottom of the screen

- Can't install search engines any more, instead you manually need to enter the correct search URL, which isn't fun (and then the manually created engine is of course lacking a proper icon)

- bookmark keywords and keyword searches aren't supported

- you used to be able to open a new tab by simply tapping the "tabs" button twice because the "new tab" button in the tabs list was suitably aligned – this is no longer the case (at least with the URL bar in classic on top mode)

- the tab queue for tabs opened from other apps doesn't exist any more

- the share sheet – you used to be able to share a URL to Firefox and then directly bookmark it or send it to another Firefox instance via Firefox Sync without having to actually open the URL in Firefox locally. Now you can't – any shared URLs are automatically opened and you need to wait for the full browser to actually load before you can bookmark/send the tab (and then you also need to close it again)

- view page source doesn't exist, not even if you manually prepend view-source: to the URL (supposedly there's an addon that works, but due to the blasted add-on policy you can't actually install it)

- can't view/browse local (HTML) files any more

- can't install themes anymore

- various small bugs introduced by the fact that this was a total rewrite

- despite having to rewrite the whole UI from scratch anyway, they didn't fix the "XKCD" bug and make long <img "title"> attributes scrollable instead of truncating them

- and probably quite a few more things I'd notice if I was actually regularly using the current version…

And of course the whole add-on thing: Even on the previous iteration of Firefox, the Webextension API implementation was somewhat half-hearted once you got to APIs that required special handling on the Android version, and the fact that this was a total rewrite hasn't helped in significantly expanding the coverage there, because instead the dev time largely had to be spent on re-implementing things.

Then there's of course that absolutely infuriating policy of only allowing a very limited selection of add-ons to be installed (unless you're on Nightly and jump through about half a dozen hoops, and even then you can only install the current version (and only the current version) of add-ons actually published on AMO). If you're lucky (or basically only using uBlock), maybe your add-on needs are actually covered by that, but the long tail of unsupported add-ons starts very early.

This also makes life more difficult for add-on developers (even if your add-on is actually one of the selected few), because you can no longer permanently install a testing version of your add-on or heaven help distribute a Beta version to your users (no, not even on Nightly and not even if the add-on is signed) – the best you get is temporary installation as long as your phone is connected to your computer and that's that.