I can't reproduce this. When I load http://jeroenhoek.nl/firefox/twice.html in Firefox 5, there is only one request made for <http://jeroenhoek.nl/firefox/dejavu_sans/dejavusans-webfont.woff>. Same for Firefox 4 and a nightly. Jeroen, can you reproduce this in safe mode?
Now that is interesting. Testing in safe mode and looking at the connections with Ettercap rather than the Firebug extension, I can only reproduce the issue when Firebug is active for that website. Firefox seems to be in the clear. As a web-developer I use Firebug almost without a second thought, I didn't consider that the very tool I used to diagnose the issue was causing it. This bug should probably be marked NOTABUG. Thank you for your help in tracking this down.
No problem. Thanks for retesting! Worth reporting this to the Firebug folks, though. http://code.google.com/p/fbug/issues/list