Wanting to (legitimately) use Firefox on a work server (to download a batch of training videos via the 'DownThemAll' plugin if you must know...) I found it to repeatedly request proxy authentication presenting me with the username/password prompt for each object on a web page.
Rather annoying as you can imagine.
Here is how to get round it, so A) i remember and B) it may help you out too...
1) Type "about:config" in the address bar.
2) Locate the "network.negotiate-auth.allow-proxies" variable
3) Set the value to "false"