Personas works even better with FF4 than with 3.x. Why have the boring blue background when you could put this there instead?

I don't mind the default blue. If there was a theme that was basically the default, but red, I would switch to that.

Because once you're used to an arbitrary set-up, it's hard to see why you would have to adapt to a different, equally arbitrary, set-up just to get other completely unrelated benefits.

Firefox has never been blue for me, always gray.

This is what FF4 looks like on my machine:
{"name":"603760","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/1\/5\/155125ecd22be9767f86e416179938ec.png","w":1280,"h":800,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/1\/5\/155125ecd22be9767f86e416179938ec"}
That's the entire screen btw; the one-pixel wide red border is the window decoration. Doesn't get much cleaner than that.

And the reason behind moving buttons around is??

Space saving maybe? From left to right saves space? It's pointless other than to try and make it look "new". If it isn't broken, I see no reason to fix it.

I don't memorize things like that, but I don't like it when my automatisms break either. Not that I'd throw a fit about it, it's merely a mild annoyance, but still one I like to avoid. I carry my .vimrc around on a flash stick for a reason.

how do you save a session?

/** Thanks. ***/

I just installed it. After unchecking "tabs on top" (hey, at least they gave us a choice this time ), the only major difference is that all the fonts look like complete arse...
[EDIT]
And that the status bar has an extremely annoying delay when mousing over links.

You could apparently disable it in the regular options menu: Options->Advanced->Browsing->Use hardware accelearation when available. The annoying status bar delay OTOH required about:configing (browser.overlink-delay).

Weird, fonts render correctly for me. Dissabling that Direct2D options didn't make any visible difference. Buggy or outdated drivers maybe?

I just installed the gzipped precompiled version from mozilla next to the official debian Iceweasel. No problems whatsoever. Fonts are equally fine.

I've switched to FF4, but the difference to FF3 is minimal. I reconfigured the toolbar to have the refresh and home buttons on the left side, and now the only two visible differences with FF3 is that the tabs are on top (which is nice, and makes more sense than the opposite), and that there is no status bar.

That never occurred to me. I just did the same and it seems to be working. It'll take a bit of time to get used to the slightly different look and feel and see whether or not it has any annoyances compared to 3.6, but so far it seems Vimperator and Firebug work (which are the most important extensions).

Edit: Any other Vimperator users notice that it no longer shows https status in the status bar? Means that if you want to know that information at a glance you apparently have to keep the navigation bar visible.

The look and feel aren't much different for me - I enabled the menu bar, moved the tabs to the top, so now I have pretty much 3.6 with prettier tab headers. Yeah, the reload / stop buttons have been moved, but I never really used them anyway.

My guess would be that you're doing something funky (read: undefined in the HTML specs) that FF 4 chooses to guess-fix differently. Not that I'm seeing much of a problem really, it seems to work just fine for me (except the iframe in the header, but it's explicitly set to the height it is displayed at, so you can't blame that on the browser).