Okay, this wasn't an obvious one, but I hope this helps someone.
My site uses a 3rd Party mobile device detection called FiftOne Degrees (http://51degrees.mobi).
This app uses some files (web_browsers_patch.xml and wurfl.xml.gz) to help browsers render the page correctly especially for mobile devices. My files were 6 months out of date and the new version of FireFox was not accounted for, causing the content-type to be set as WAP instead fo HTML.
So I updated to the newest versions of the files (from http://wurfl.sourceforge.net/) ... and it actually didn't fix the problem... BUT I've checked the "last updated" dates in these files and the last time they were updated was before FF4 came out, so I'll have to wait for the newer version of these files to come out (or maybe hack the files up a little to account for the updated User Agents).
For the time being (on test), I've just switched off the mobile device detector in the web.config.
Hope that helps someone.
Thanks everyone for the help!
EDIT: I raised another question to find a solution to this out of date WURFL. There's a few work arounds in there. Check out: WURFL not detecting FireFox 4 successfully