This is exactly why anyone that says they are a web designer - yet does all their work in some 'pixel perfect' tool like Macromedia (or whatever the 'cool kids' use these days) is just deluding themselves.
Turning off the stylesheet on every page as I browse the web would be a hassle; much better to change a setting once and never have to worry about it again. About the only pages I've found that aren't improved by enforcing a sensible font-choice are pages designed to showcase CSS3 font embedding - and those are pretty rare.
I do wish browsers would have a per-host "allow custom fonts" setting, alongside the tradition per-host "allow images" and "allow cookies" settings. With the potential security implications of downloadable fonts, it seems only sensible - but nobody seems interested in implementing such a thing. :(
Untick that box, set the Firefox defaults to something attractive like Droid Sans, and rejoice at how readable the Web becomes.