This question is because I just found out that my site is looking ok in IE7 and in IE8 with compatibility-mode, but in FF it is all screwed up.
What would be the bes
There are often ways to make your css compliant with both browsers. This takes a little bit of time, but makes it easier to maintain things later. So if you have the time, I recommend trying to find ways to implement all elements in that fashion.
I often run into this when centering objects. Most recently I was specifically having problems with a table for a web-portal. Googling "css centering table" yielded table centering using html or css. Using this same approach you should be able to figure out how to reconcile the browser discrepancies.