Firefox OS User Agent
While we consider user agent sniffing a horrible practice on the client side, UA sniffing is done quite a bit on the server side to serve up the appropriate page version of a site, or redirect to, for example, the mobile version of the site. This can be a dangerous road but most large site with a separate mobile interface do it. The following is the user agent for Firefox OS:
Mozilla/5.0 (Mobile; rv:18.0) Gecko/18.0 Firefox/18.0
At the moment, many large sites are sniffing for iPhone|Android instead of "Mobile", and thus they are serving their desktop site to Firefox OS devices. Be aware of the Firefox OS user agent and update your server side logic appropriately!
Note: Of course the version number within the user agent will update often, so detect "Mobile".