[Update 2011-06-25: You should consider this post obsolete. See this post for more info. Iceweasel 5 installs most beautifully on Wheezy.]
I just tried building Firefox 4 on Debian Wheezy. I build Firefox for my machines because (1) I want the latest FF and (2) I want subpixel rendering--which tends to get turned off sometimes|always in Debian and Ubuntu builds of Firefox and Iceweasel.
My first attempt failed. Why? Because if you enable-system-cairo in the build, FF4 insists that you have at least version 1.10 of libcairo2--and Wheezy and Squeeze are both at 1.8.10. One upshot of this is that there will be no way for Squeeze users to get subpixel rendering in FF4 using the default libcairo2 packages. Wheezy users will have to wait until the current Sid packages trickle down. Once that happens, it theoretically will be possible to backport the Wheezy libcairo2 to Squeeze (with patches even) , but I am frankly getting a little tired of this.
I am now making a second attempt to build FF4 wherein I did not enable-system-cairo, and it seems to building just fine. That should be ready for testing in another hour or so. But of course I expect font rendering to be just tragically bad. (Update: it built and runs fine and renders fonts every bit as badly as I expected.)
Tomorrow I will probably try building FF4 on Ubuntu 10.10, whose libcairo2 is at 1.10. I tried running the Linux prebuilt binaries from Mozilla on my Ubuntu machine, but as I have come to expect from Mozilla's FF builds, cairo did not seem to be enabled. (Update: Building on Ubuntu also failed. Apparently FF4 with enable-system-cairo also needs cairo-tee greater than 1.10, but Ubuntu's libcairo2 doesn't seem to be built with --enable-tee=yes.)
It seems every other post of mine in the last couple years has dealt with the agonies I've had in getting consistently good font rendering in Linux. It really shouldn't be this difficult.