I use Droid Sans Mono as my font for coding and in Putty and nearly everywhere. You can read and download it over at damieng.com.
If you want code and fixed-width text to appear nice in Firefox, what’s a better idea than using Droid Sans Mono there, too?
Go to Options > Content. Under Fonts & Colors click Advanced and choose Droid Sans Mono as monospaced font. It’s as easy as that. It will now affect every fixed-width text that is not formatted by CSS or something else. It will even change fonts in Firebug. I like it, it looks beautiful!