A few years ago Mozilla chose to focus on version 7 of the ARM platform instruction set (ARMv7), as it was the only one that provided the necessary performance to deliver a good Firefox experience, and much less crashing prone.
Fast forward, and as Ted Mielczarek, Mozilla build expert, explains, Mozilla has both now: a stronger mobile team, and a more complete picture. It turns out that about 60% of Android phones in use right now are ARMv6-based, which means 60% of Android phones can’t get Firefox. So it makes perfect sense to invest in having Firefox serve such a large mobile audience.
The ARMv6 instruction set is implemented by the ARM11 microprocessor family, running on a long list of well known, if lower end or a generation behind phones including the Samsung Galaxy Mini, Galaxy Ace, Amazon Kindle 2, HTC Touch, Diamond, LG Optimus One, S, Sony Ericsson Xperia X1, X2, and a very long etcetera. Check Wikipedia’s entry for ARM11 for a more complete list.
Ted doesn’t mention it, but I am guessing the decision to have Firefox use native Android widgets, as opposed to Mozilla’s own XUL-based ones, is also helping to ease processor requirements.
In this scenery, and just like on Windows, Firefox would become more backward compatible than the OS maker. Chrome for Android beta is at this point only supported on Ice Cream Sandwich, Android’s latest version. (By the way, Chrome also has a crazy country-based restriction!).
If you are interested in the subject check Ted’s nice and detailed post.