But then those developers will develop for that new phone, creating a bunch of stuff that will not run on the older phones.

Since those older phones are Mozilla's target market it would be an odd choice to make.

It's a little bit like the WWW where people (rightly) develop on very powerful hardware but often (wrongly) fail to test on a range of normal hardware. (This is also ignoring the fact that stuff written for the WWW should not need testing because it should all be standards compliant etc).