Firefox has "Ubuntu" placed just before "X11" while Chromium has it placed later on in the string (after "(KHTML, like Gecko)").
There have been instances of web sites not recognizing Firefox only when the distro is Ubuntu: in two cases (http://www.summitracing.com/ and http://translink.com.au/), it appears that the web master fixed the issue somehow. The third instance (http://www.property24.com/) wasn't yet fixed when I last checked.
In the first and third cases, the page concerned didn't open and users were asked to download something like "Mobile.aspx". In the second case, users got mark-up meant for mobile platforms.
Users of other Linux distros didn't have problems and those using a version of Firefox downloaded directly from Mozilla didn't have a problem either.
Users of Chromium on Ubuntu didn't have the problem either.
At the time of posting, http://www.property24.com/ still offers to download Mobile.aspx and doesn't open the page correctly if "Ubuntu" is present ahead of "X11".
If I tweak the user agent string to Mozilla/5.0 (X11; Linux i686; rv:11.0a2) Gecko/20120114, Ubuntu, Firefox/11.0a2, the page loads without issue.
The "summitracing" issue was reported here.
Edit: bug 89774 relates to issues arising from introduction of "Ubuntu" into the UA string. A "patched" beta Firefox is available for testing. Details of the ppa are available at Chris Coulson's blog.