And, if I remember correctly, the whole Profile things was initially hidden in Firefox. I remember you couldn't easily switch profile without using the command line.

Yes, it was there, but not really supported. We had to (ab)use the feature for some stuff, but everything was read-only and locked down by enterprise profiles, so it was not a big problem for us.

I think profiles are not supported as first class citizens, still.

Shall that be improved? Yes. Will that be easy? I don't think so.

If they have the resources to constantly revamp the Firefox and Thunderbird UIs, they have the resources to rewrite the profile system.

UI reboots are so sexy though -- if this is like any company I've been involved with, they hire/promote a new design boss every couple years, and that person spends 6 months making a sexy Illustrator/Sketch/Figma (in that order over the years) document showing how hott things would look if we changed all the fonts and aped whatever widgets had been redone in last year's iOS. Instant greenlight. Try getting that kind of C-suite buy-in for a boring under-the-hood fix!

This constant redesign with no real improvement cycle irritates me to no end.

Actually Thunderbird Supernova and Firefox’s latest iteration all very good both in space efficiency and performance.

I’m saying this as a lover of 90s dense interfaces, like Eclipse.

Did you see the code? Do you know how deep it goes? I don't have the definitive answer, so if you have some insight, I'll love to hear it.

UI is top of the proverbial iceberg. It's relatively easy to modify it.