Font fallback Firefox extension

Is there an extension for Firefox that allows for quick switching between the different fallback fonts specified in CSS?

It's quite annoying having to do it by commenting stuff every time. I'm too lazy to do that, and of course too lazy to write one myself.

Also, anything that makes me see Arial for extended periods of time is bad when I have Helvetica (stole it and every other good font from a hacked OS X box, now I don't need a Mac anymore), switching manually takes too long and might hurt my eyes or brain.

clearly the point of this point is to tell you that you're too damn lazy. there's nothing that does what you want automatically, so just do it manually.
however, a little tip; try setting up seperate external stylesheets with the different settings you want. that way, you only have to change the name of the file it links to instead of fucking around with commenting things out.