Carlos, there are loads of reasons to use firefox in the development cycle of a page.
It might be that 90% uses IE where you live, but this is not correct for for instance Europe,
in some EU countries the FF marketshare is around 30%.
See link01, link02 and link03
Also FF is much better at handling the standards then IE, even version 7 (IE6 is IMHO the worst "modern" browser around, it's loaded with CSS bugs).
If you want to use W3C (x)HTML/CSS you should develop your sites for FF or Opera and then add some hacks to make it work in IE.
People who develop for IE tend to get used to the incorrect CSS interpretation of IE and complain about FF rendering their page incorrectly...
Also there is a webdeveloper toolbar for IE, but it doesn't have more then 30% of the functionality of FireBug, the webdevelopers extension for FF, I couldn't live without it.
Don't get me wrong, I really like some of MS's technology (like .NET;-) but for their browser there is a much better alternative.