Firefox indexes Fonts

While I was looking for a particular song I heard on the radio on several top-whatever websites I got this Firefox warning;

Firefox wants to use the font "HelveticaTStd-Roman" on the volume "xxx"

A couple of remarks;

• Why is Firefox indexing the fonts on my system? This particular font sits in a directory not used by my fontbook.
• Fail on the part of the top 40 website for using a very specific (and obviously) non-default font in their design.