It all happened all of sudden on Gmail first and the other sites too after I checked more closely. I did a quick Google search and someone mentioned that it's because Firefox 3 somehow screwed up the Windows System fonts, Arial in particular. And it was suggested that in order to fix I need to manually delete or remove the fonts and reinstall it. In my case, the font Arial even went missing and I could find it in Fonts folder at all.
So I downloaded a System Arial font for Windows which fixed the problem perfectly after I manually installed it back into Windows Fonts fold.
It's still interesting though what exactly Firefox 3 did to make it happen and I am also surprised that there were number of people having the same issue.