Windows Firefox VS Linux Firefox

I had read several times before that firefox for windows and linux is derived from the same source code, so they should render windows absolutely the same. However I haven't found this to be the case with one of my sites. I just installed Ubuntu, which by the way is pretty freakin sweet (my laptop never gets slow, never hangs up, and my battery lasts 2x as long!) and firefox is showing my site a bit differently than it did in windows. Now certain lines of text that used to fit on one line fill 2 lines.

So what's the deal here?

Please provide screen captures of both for us to see what you are talking about.

Are you running the exact same resolution and default font sizes?