Often when I'm reading up on support for particular HTML or CSS features, I'll see something along the lines of "Most modern browsers support this feature, Internet Explorer doesn't". A loose translation of this could be "if you use this feature it won't work for 80% of your users".
I do most of my testing in IE then go over to FireFox to make sure it looks OK and probably because of this it always seems to be the other way round, FireFox doesn't support a lot of stuff that IE does.
FireFox doesn't have a way to render text vertically. OK, there's no official way to do this, so IE has come up with it's own CSS attribute, but I'm kind of surprised that FireFox hasn't come up with some way of doing it, it seems like quite a common thing to want to do.
Another problem is hiding and showing rows in a table. FireFox kind of supports using 'display:block' but the row gets taller and taller with each hide and show. So I have to use 'display:table-row', which doesn't work in IE. Not sure which is standards-compliant but 'display:block' seems like the more consistent approach.