I agree very much with kb57. The squared-off "sharp" tab edges look very unpleasant. And I was at a loss where to find Refresh at first.
Admittedly Fx is a lot faster, as they claim, in fact mine goes like a rocket.
But the most annoying thing, personally, is the possible abandonment of certain "legacy" extensions in Add-ons. I'm used to having some protection from malicious scripts via NoScript, though the guy who created it promises a new version "soon, perhaps by the end of the day", but he's been saying that every day.
Being the acquisitive type, I also have a lot of web pages downloaded via UnMHT, as single files with the .mht extension, and it seems likely it won't be updated. Now I can't read any of those saved pages, so having UnMHT turns out to have been necessary. The .mht extension is Microsoft's, so it ought to be easy enough to access the pages, but no luck so far. Fx is supposed to do it, but doesn't. Will have to look through
this list and find something.
Finally, there was machine-resident Video DownloadHelper which I used a lot, but wants money now. Fair enough, the developer could reasonably expect to be rewarded for their efforts, but I just don't fancy parting with money online when I know nothing about the vendor's trustworthiness or security precautions.
There's still 4k Video Downloader for YT vids, but it doesn't work on other sites, and there are signs it won't stay free.
Very extensive and "self-entitled" grumble over! I suppose it'll all work out in the end. Birders have to be perpetual optimists...