There’s an excellent article by rakaz on the new themes in Firefox and Thunderbird 2.0. I agree with the whole thing, in particular the strange and increasing amount of seemingly Mac-influenced design decisions for a world which is primarily Windows XP based.
To me, such decisions seem like short-sighted mistakes. I still, for example, can’t get my head around what kind of decision-making process went into creating the new navigation in the Firefox 1.5 options pane. Forcing them to adopt entirely new (and application-unique) UI metaphors is not a way to embrace people migrating from any other browser.
Anyway, there are screenshots and comparisons of the UI changes in the article, so go and read it!