Hmmm...well, that's not what it's looked like for me since I can remember...I've reinstalled Vista in the past month and when I did it was at a different font out of the box. I don't understand it...just means I'll need to get used to this font now. When I use Ctrl+Mousewheel (to change font size) I've found one that looks like what I was used to, but I don't think it's exactly the same. Meh, probably PEBKAC. Thanks.
In Firefox menu go to Tools -> Options then open the Content tab and in the Fonts & Colors section click on the Advanced button.
The default settings for western countries are like these:
Set the settings like that and make sure that the "Allow pages to choose their own fonts, instead of my selections above" option is ticked. Then close and restart Firefox.
As Lateralus said the default font for this forum is Verdana so check in the Windows Control Panel -> Fonts if this font is installed and if the files are physically present in the \Windows\Fonts folder (4 files: Verdana.ttf, Verdanab.ttf, Verdanai.ttf, Verdanaz.ttf).