Graphics problems after upgrade to 5.5

After upgrading my laptop to Trisquel 5.5 I have experienced problems related to X and/or the graphics driver. These problems prevent me from using the graphical interface, so I've had to boot off of a 5.0 CD to post this.

It's a x86-64 laptop with a Radeon HD 6520G graphics card. It works in 5.0 but for some reason it can't be configured properly under 5.5.

Here's the output from running `startx` manually: http://ompldr.org/vZG4xMA/startx.txt and here's Xorg.0.log: http://ompldr.org/vZG4xMw/Xorg.0.log. The error in question appears to be:

(EE) Failed to load module "fglrx" (module does not exist, 0)
(II) [KMS] drm report modesetting isn't supported.
(EE) RADEON(0): Chipset: "SUMO" (ChipID = 0x9647) requires KMS
(EE) Screen(s) found, but none have a usable configuration.

It's my understanding that fglrx is the proprietary driver, but of course I'd rather not install that. I've looked for solutions but the recommendation just seems to be to use the proprietary driver. Running an `modprobe radeon` simply causes my screen to crash entirely. Is it possible to get this chipset working with the free radeon driver? My kernel is 3.3.3-gnu if that helps.