How do I block a certain font on Firefox ?

I’m on a Mac right now.. Windows should be about the same.

Preferences—> Content—> Fonts and Colors—> Advanced

You can figure it out from there.. And I know this doesn’t totally answer your question.

that was actually the first thing i tried, but unfortunately the only workaround using the advanced option is to uncheck “Allow Pages to choose their own fonts” which causes other sites to look bad since I can only use the four basic fonts

I’m not sure, but I think that you could open about:config, search for “font” and then look for a few keys whose value is “Helvetica” (e.g. I have font.name.sans-serif.tr). I suppose that’s those set sans-serif fonts to Helvetica by default and to Arial if you change that.

@vincent: i tried that too. The default san serif font is still Arial. It seems that since I am allowing sites to choose their own fonts, they are using the designated Helvetica before Arial