There is potentially a critical difference here. The "We don't support IE" mini-movement was a reaction to Microsoft's refusal to use the web standards that every other major browser supported. In this case, it appears to me (admittedly without research) that Google's motivations must be different, and two possibilities spring to mind: (1) Google doesn't like the competition from Firefox or (2) the Play store uses non-standard features not supported by Firefox's overwhelmingly standards compliant browser. If either of these is the case, we've got a very different situation, potentially one in which Google has begun to play the part of Microsoft, implementing and using their own non-standard features, even if it's in a less intrusive and less inconvenient way.