Is there any way to make page inspector work on browser chrome to see xul tree/css? Searching on the internet for "firefox chrome inspector" is just ridiculously frustrating because of naming conflict with the other browser.

Hmm, looking at css, it's still possible to toggle browser.proton.enabled to false, to get at least some of the old UI back (tabs will be less crappy), but the new "color" theme will persist.