Different shortcuts, ways of customizing the browser, etc. the browser may feel like second nature to you currently, but for others, there's friction in changing the software you've use for over a decade, and I say this as a current Floorp user
I guess a more honest way to phrase it is "people who are unwilling to learn a new browser", since there's nothing specifically difficult about Firefox.
Let the blame rest with the user that won't learn, and not with the software they refuse to learn.
I use Floorp and JUMPED to it from Firefox because I had a mediocre Firefox experience. I fancy myself a power user and was not a fan. The idea that the majority who try Firefox and have issues are in the wrong and the minority who enjoy the experience are right seems backwards...
I'll be honest; I bounce between several browsers - Firefox, Chromium, Chrome, and even sometimes Edge, and sometimes it takes a second for me to even remember which one I'm looking at. Firefox is great for very specific work flows I have, but for a lot of other things, most other browsers will do.
Maybe it's because I tend to bounce around that I find it very interesting to hear that FF is difficult to use.
What shortcuts are different? Basically most of web browser shortcuts are universal, e.g. Ctrl/Cmd + L to focus on the URL bar, F5 or Ctrl + R to reload, Alt + Left/Right arrow to go back/forward, Ctrl + D to bookmark, Ctrl + T to open a new tab, Ctrl + W to close a tab, etc. I've been using these for decades across different browsers, god damn they even work in Apple's Safari
There is absolutely nothing difficult to use in Firefox. If anything it's easier, as most of the settings aren't arbitrarily hidden to prevent you from changing them. There are also fewer bullshit settings because they aren't harvesting your data.
I'm not a user of brave, but I did a quick Google and it looks like they're ad blocking will be unaffected. As for other extensions, I think that at least some will be supported for a year, while others may break immediately but I didn't take too deep TBH