Firefox asks for proxy details, but only when accessing facebook

Most probably, your proxy is blocking the WebSocket protocol. Chrome before version 14 and IE don't support it, so they are not affected.

Go to the URL about:config, search for the key network.websocket.enabled and switch it to false.

I Just tried the suggestion in the accepted answer and I'm afraid it didn't fix my problem, as well as these other suggestion which also didn't work: http://nasarabna.wordpress.com/2010/03/03/firefox-keeps-prompting-for-proxy-authentication/ (These solutions may be currently outdated)

However, while browsing the aboug:config page I found this entry:

signon.autologin.proxy

If this is set to TRUE the prompt does not appear anymore.

BTW: It does only work if you log in at least once. If you cancel it, it keeps appearing.

And, as a consequence, if you use the private browsing feature (I use it at work, for personal stuff: e-mail, etc.), every time you start a new private session you have to accept the login prompt. Not a big problem though...

I've been having the same issue for a while now. It's a bug specific to Firefox, and only when accessing Facebook addresses through a proxy. This page itself, for instance, brings up the dialog a total of 10 times, taking 10 clicks before I can dismiss it and go about my business.

I have since downgraded to Firefox 4.0.1, which fixed the issue. It's not a great workaround, but it's there.

EDIT: It turns out that last bit was premature - I've now started experiencing the same problem on 4.0.1.

Firefox 8 added Origin tag for images, which has caused some specific proxy issues. If Facebook works for you with Firefox 7, I would say it's a bug in Firefox.