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>>Yes, I added it later. The FF is my primary browser. I use IE only if I have to.
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>I also noticed that FF renders my HTML code differently than IE. The HTML code is created in ASP.NET page so this could explain it. I may start using FF more as I think it works faster. But I hope my customers will stick with IE, otherwise, I will have a tough time making my pages look the same in both browsers.

I think it's a general practice to try to test the application in as many browsers as you can. Otherwise you would need to put a disclaimer...