A "fix" for this was suggested during alpha stages of FF3, a suggestion to put in several modes of zooming instead of just using billinear scaling (or whatever it's called they're using now), the response was "Why would anyone want that?".
Yeah, it's not needed.. Doing the following should make everyone happy:
For image zoom <200%, do interpolation / decimation.
For image zoom >=200%, don't interpolate.
This is roughly what the latest version of GIMP does. It actually annoys me a little when I get to having say 150% zoom, but you guys'll only want multiples of 100% anyway.
I can confirm it seems to only happen on Windows. (ie. I'm using FF3.01 on Ubuntu Linux to post this, with no blur when zooming.)