Firefox 3 was having some font rendering issues on my machine. Certain web sites were displaying a wierd bold looking font in place of the default font described in the CSS. See this screenshot. After doing some searching around the internet I found out the issue.
Early in the year I had installed Adobe Type Manager and added a bunch of Helvetica Postscript Type-1 fonts. These were for some reason being used as Firefox's default font. Removing them from the system fixed the issue.