It looks to me like you have a difficult choice to make:
1.) Take the time to get your site cross-browser comaptible, which is likely to be a long process with quite a learning curve, or,
2.) Accept the fact that non-IE users (around 20% of Internet traffic) will see your site in a broken state.
Only you can make this decision, based on the time you have, the goals for your site, etc. Some people here advocate complete cross-browser compatibility, which is great if you already know how to do it. Others advocate making your site work for as many browsers as you're conveniently able to do, if your time could be more profitable if spent in ways other than achieving complete cross-browser compatibility. You'll need to weigh the pros and cons and make your own decision.
A few pointers:
1.) Validation, though extremely helpful, does not guarantee a completely cross-browser site.
2.) You'll need to use CSS if you want to use valid markup that renders the same cross-browser
3.) It's easier to develop "for" FireFox and then debug for IE, than to do it the other way around.
4.) Don't count on your statistics to give a reliable representation of what browser traffic you're getting - if FireFox users get a broken page, of course they won't visit more of your pages or come back, thus "artificially" bloating your stats in favor of IE when it comes to actual visits and visitors.