"The chrome seems laggy and unresponsive at best."
That is one of the problems of making an entire interface out of XUL and JavaScript and never restart from scratch for over half a decade...

"The new text/text shadow rendering looks awful, especially with a larger font sizes."
That is actually Window's fault. Direct2D uses different text rendering settings than GDI (which is absurd at minimum). For Firefox to have hardware accelerated content on Windows, it will necessarily have that text rendering. I think it is possible to change D2D settings to look uniform with the system tough.

"They’ve removed the RSS button."
Actually, they made it a toolbar button, you can now place it anywhere (although I do agree it should have been kept visible in the default layout).

My opinion is that I hate to admit it but I'm forced to agree, Firefox needs to make a clean up and polish revision. An example of the benefits of this is how the new HTML parser (made for HTML 5) solved an incredible number of bugs, because it was made from scratch, instead of trying to fix what Mozilla had before.

Other than this, I don't know what more to say. Firefox is my favorite browser, it seems to me to still be the only one to take a somewhat native approach when making an interface (FF3 interface on Windows Vista is not the case because they weren't able to include Aero support), while all other browsers go with interfaces which are incredibly small (or not), but also incredibly heavy on the eyes. It also seems to be the most customizable browser and I like most features of it. Yet, I see all other browsers, quick and responsive, but I still don't like them. I don't understand why. Nonetheless, I'm getting tired of Firefox' quirks.
It's hard to explain, and frustrating to feel it.

(One thing for sure: firebug beats Webkit developer tools anytime)