If UX designers are designing unguessable interfaces, then they’re not doing a good job. A good UX designer would make it better than guessable, they’d make it mostly intuitable so you don’t have to guess for most things. As much as I’m against Google and that I use FF, Chrome/Chromium (and derivatives that don’t change too much of the UI) does have the best UX (IMO) with Safari as a second, amongst the browsers I’ve tried. I use FF in spite of the UX because I align with their goals and values.

With that said, good UX is REALLY difficult and, as a non-profit, I give FF a pass. I give them more of a pass because UX gets more difficult when you also emphasize customization and options. However, it’s one area they should definitely work on if they want to capture more of the market. Most people I know use Chrome because it’s “easier to use”.