You may be better off asking that question at the Mozillazine forums [forums.mozillazine.org], where the Firefox guys hang out. I don't know much about the Linux/Mac version of the browser, but I gather that there is a separate gtk2/xft enabled build. It might be worth seeing if your Firefox has been compiled that way, and asking in the builds forum for a recent non-xft build.
Most recent builds are listed at Burning Edge [squarefree.com]. It should be noted that unofficial Mac builds are rare, because there aren't many people on the Mozillazine forums who use Mac OSX. You may actually have to compile your own :-/
When posting in the forum, you should copy and paste the build data from the Help>About dialog into your message so people know what you're using. My build is copied below as an example.
Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.7a) Gecko/20040220 Firefox/0.8.0+
