Mariusz Mlynski reported that if you could convince a user to hold down the Enter key--as part of a game or test, perhaps--a malicious page could pop up a download dialog where the held key would then activate the default Open action. For some file types this would be merely annoying (the equivalent of a pop-up) but other file types have powerful scripting capabilities. And this would provide an avenue for an attacker to exploit a vulnerability in applications not normally exposed to potentially hostile internet content. 

References:
http://www.mozilla.org/security/announce/2011/mfsa2011-40.html
https://bugzilla.mozilla.org/buglist.cgi?bug_id=657462,662309,663899