Cannot browse any stackexchange sites using FireFox without clicking through 5-10 "moz-proxy" popups on each page

firefox

I am experiencing a very annoying problem with FireFox 11.0 on Windows XP SP3, and unfortunately it is making it very difficult to use one of my favorite sites!!!

This problem appeared several weeks ago. Anytime I navigate to a Stack Exchange site (i.e. this site, SuperUser, but also StackOverflow, ServerFault, etc.) I am presented with 5 - 10 of the following pop-ups - on every page:

moz-proxyfail

A few things to note:

This pop-up does not prevent me from viewing the page. It is just very annoying. I can enter in my username / password to no effect, but I also can click cancel repeatedly. After about 10 tries, I finally can read the page (but have to start over on the next page I view).
This problem doesn't occur in IE.
This problem does not occur with any other web sites, just StackExchange sites. Why?
When I do enter my AD user / password, FireFox pops up the "do you want to remember this username / password?" popup (see here), but is immediately dismissed by the "moz-proxy" pop-up.
I am not using (nor do I want to use) any sort of proxy add-on like FoxyProxy.

Please help as I want to be able to use my favorite developer site whilst at work again :(

In order to make the dialog disappear, I had to switch from "Use system proxy settings" to "Manual proxy configuration", copy proxy settings from IE, and add 'sockets.ny.stackexchange.com' to 'No proxy for' field.

Another option is to install 'BlockSite' add on and add 'http://sockets.ny.stackexchange.com' to the blacklist.

The most likely explanation is that the proxy server you use permits the stack sites, but doesn't permit some of the third party elements that make up a page, such as adverts and any other external resources.

Each one of these that is blocked returns a permission denied to Firefox, which causes firefox to prompt you for different credentials to access the blocked content. IE doesn't do this, if it gets a permissioned denied from the proxy server, it silently drops the content (you will sometimes see your proxy server "blocked" message appearing in the middle of pages where this has happened in IE).

To test this, look through the source of a page in ff, and search for any instances of http:// then try each of these URLs in IE to see if you get a proxy blocked message.

If you do, you can stop these URLs being presented to the proxy by putting them in your c:\windows\system32\drivers\etc\hosts file: