Trying to update the firmware on Zoom 4506 and 4501 mouter routers fails if you run it from Firefox 3.6 on Ubuntu Linux. It appears to load quickly, then stops with a 500 Server Error error message (from the webserver on the mobile router).
The solution is to use a different browser. Google Chrome works. The upload takes much longer (pauses for a while at 32% loaded) but it does complete OK.