In both the MobileDevice table (built-in) and AppConfig:MobileShortUserAgentList (Mobile Commerce Plug-In), there's a string called "eric."

That string is responsible for mistaking Firefox for Ubuntu (desktop) as a mobile agent. The reason was:

Mozilla/5.0 (X11; U; Linux AMD64; en-US; rv:1.9.2.3) Gecko/20100403 Ubuntu/10.10 (maverick) Firefox/3.6.3.