I *very often* (nearly always) *do not have* GSM file in that list, on a fresh install of Firefox. And if the type of file you have isn't in the list and you aren't presented with the "open with" option, then you're screwed.
I know that's not supposed to happen, but it does, and not infrequently.
Additionally, the dialog of which you speak -- and this complaint is closer in spirit to the sort of thing they're actually looking for --
1) has 2 columns, with an immovable divider
2) cannot be resized
3) lists the human readable name of the filetype (which is almost always useless) followed by the mimetype (which is very often useless, and is often clipped off partially or completely because of 1 and 2) and does not show the only thing you can actually see -- the file extension -- at all.