I can't see why Gecko/Gaia won't have the same problem, looking at the architecture docs.
My (fairly naive) understanding is that there is nothing comparable to the Gecko layer on Android (Dalvik seems in practice to be very intertwined with lower level code) so I'm not sure what conclusion you're drawing from the docs. Feel free to school me on it, though. :)
As for equivalence, gecko, among other things, has the drawing layer.
At least according to https://wiki.mozilla.org/B2G/Architecture, Gecko is using OpenGL ES 2.0 to do drawing.
This means it will be impacted by graphics driver bugs and issues, the same as Android is.
(Which is not surprising, since Gonk, the layer below it, started out as Android)
Look at it this way: anything that lives in Gonk will generally have exactly the same issues as Android does in terms of updates. There may be some pieces of it that are more generic than others (bluez, for example), but by and large, you will need to test on each phone model.
FWIW, the Gecko layer is essentially the same as Android's framework.
What functionality is provided by a high level layer differs a bit, but Android and Firefox OS are not as architecturally different as you seem to think.