Revisions and Commits
FF says it uses DejaVu Sans system font. This is <pre></pre> area, so only monospaced font should be used.
As I understand from http://en.wikipedia.org/wiki/DejaVu_fonts , only some DejaVu fonts are monospaced (ex. DejaVu Sans Mono), and one used by FF isn't.
This looks like a bug in FF. I am on FreeBSD, you might be on different system, so behavior is different.