On a whim I've just bought myself a ZTE Open C, it's actually a lot better than I anticipated. In all honesty I actually expected Firefox OS to be a bit stuttery but so far (and it's only been a day) everything's been really rather smooth. It's miles better than my daughter's old Samsung Galaxy Europa and not too far behind my Huawei Ascend Y300.

One thing that's very noticable is when you launch the Firefox browser it loads instantaneously (presumably because it's already in RAM as part of the OS), I like that a lot.