Maybe, all things being equal, that would be a fair complaint but all things are not equal.
First of all, the limitations of iOS and app store are well known and retread at least once a month on HN since the site opened. Mozilla certainly did complain about iOS years ago and decided ultimately to abandon the platform. We're talking about Windows now because Windows 8 is due to be released.
Secondly, Firefox's primary platform is Windows and they already have working version of Firefox for Windows 8 with a Metro UI for x86. Only a minor technical limitation keeps them from releasing it for ARM -- that's not true of iOS.
Lastly, there is a still a chance Microsoft might cave on this issue. We know from years of experience that Apple won't.