Need Help: Troubleshoot Layout Issues in Firefox, Possibly Other Browsers

[PixelHippie]

Hello Sitepoint community! First time here so a small introduction is in order. The name's Austin. I'm just starting out in the wonderful world of front-end development.

I'm still very fresh in web design and humbly come seeking wisdom and expertise.

I'm actually finishing up with my very first client. It's a basic layout and design for now but will expand as my client's business does. HTML5 & CSS3. The site works flawlessly in Webkit based browsers as well as Opera and IE9. Yet, for whatever reason, Firefox just isn't playing nice.

I lack the experience necessary to troubleshoot this. I honestly don't know where to begin. I'm not even sure I can accurately describe the issue...so, I pushed everything live in the hope that someone could peruse my code and at least tell me where to begin or what to scrutinize. I'm not looking for a handout. I love web design and coding in part because I take great joy in solving such problems.

You can find the site at http://lvthreshold.com.

I can provide screenshots and code if needed. I've been running it through validators and cross-browser tests. I'm personally using Firefox version 3.5.5 on OSX Lion. Written using Coda for Mac.

Thank you in advance!