Currently there is a report  of the Motorola ME632 (http://www.gsmarena.com/motorola_me632-4272.php) [640x480] that was once compatible with Firefox and now not on Google Play with the November 8th update (17.0b5). This is possibly due to the change https://hg.mozilla.org/releases/mozilla-beta/rev/77455622c8ac on mozilla-beta from bug 805191 What would make this device now incompatible by manifest changes?  https://groups.google.com/forum/?fromgroups=#!topic/mozilla.dev.platforms.mobile/PxhfrkLLiGw
Thanks for filing this bug report, Aaron. If there's any information I can provide in order to solve this issue, I'd be glad to. Regards.
This should be fixed now with small-screen support added for Firefox Beta.