On my own sites I see IE has the largest share but not by too much. IE is ususally between 45% and 50% with FF being 37% - 42%. (All my sites are tech related).

I'd suggest working on your sites till you get them working in the two main browsers.

I have seen some sites use a seperate CSS file for IE... so it might be worth you looking down that route if it's CSS problems you are having with layouts etc..