I figured out a solution.
The issue is with how FF renders <div> on different platforms. I ended up using: -mos-margin-start and -moz-margin-end (effectively margin-left and margin-right) to control the div.
The PC version didn't need the margin controls so I used a browser & platform sniffer to implement an Apple only style sheet.
And of course this didn't work 100% on the Mac platform either. While I was able to stop the incremental change in FF, the difference between Macbook Pros with the exact OS and FF version rendered the div 1px off. We'll live with this one as I think the value of trying to accommodate FF has reached it's limit. Our traffic for FF on a Mac is about 2.4% and this minor difference between Macs isn't worth chasing.