It looks like one of the most recent Firefox updates has broken Google Gears' compatibility with WordPress. As of a few days ago opening the media uploader seems to cause a total browser crash.
I can't find any way to work around this issue short of completely disabling Gears. Even deleting your WordPress installation's profile in Gears doesn't seem to do the trick.
Just to make sure Gears is the culprit, I tried disabling every single other Firefox plugin and every single WordPress plugin. No dice.
I don't know what needs to be done to resolve this, but I'm fairly sure this is an actual issue and not a localised configuration issue because it's causing the same crash on two different computers, operating systems, and on all WordPress 2.7.1 installations (I operate many).