Consider if a theme could accomplish what you want: It seems like they all use firefox under the hood, and if all you want is appearance, you shouldn’t need to change your entire browser. Keeping in mind that a smaller fingerprinting pool is less anonymous, if you care about that.
I haven’t actually used any other than librewolf though, so if switching provides any features you care about, go for it.
Do you know of any zero-knowledge providers that are both (a) trustworthy for my own purposes, and (b) unlikely to go to spam?
Like you said, the incoming messages aren’t encrypted, so “zero-knowledge” is always sort of false advertising. Also, if I have to use some weird client, that isn’t good. I do value convenience, especially for email; chasing diminishing returns just isn’t worthwhile, and if possible I’d like to not use both, as I am now.