Some notes and things I've found:

• I created a completely new Firefox profile with no add-ons and everything allowed to get the above results.

• Thanks to the quick feedback comment, I learned my Chrome browser passes all the tests on the self-help and diagnosis webpage linked kindly by @Arjan: http://meta.stackoverflow.com/network-login-help

• My Firefox browser also passes the above self-help diagnosis webpage

• In Internet Explorer 8 (IE8), I'm able to login.

• I can successfully log into https://openid.stackexchange.com/account/login using Internet Explorer, Chrome, or Firefox. This login session can't be used to access other Stack Exchange sites.

• We have a proxy at work, though I have no access to its settings.

• At home I'm able to log into any/all sites without issue.

It seems there's something different in the way Stack Overflow sites handle the Chrome and Firefox engines, otherwise we wouldn't be able to login with Internet Explorer.

It also seems the openid.stackexchange.com site handles logins differently than the other SO sites.

Though this problem is unique to our work environment, there's enough things in the way the SO back-end is set up to persuade me to post this. If others are not experiencing it now, they likely will in the future.

If there is something I can do to gather more information, like doing a Firebug or Fiddler capture, please let me know - though I'll need to be told what to configure and how, and what to look for.

I look forward to either hearing something I missed doing/not doing, or learning something neat about the SO login process that allows me to login using Chrome or Firefox, or learning there's a configuration setting on the Stack Exchange side of the house that could be tweaked to allow me (and others?) to log in using Chrome and Firefox again.

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See the "what else" in Which browsers are officially supported? And what else do I need?, especially the automated Network Login Troubleshooting tests. –  Arjan Dec 26 '12 at 20:27
It looks like your work proxy is blocking the openid.stackexchange.com/affiliate URL path thinking it's an ad...which is just a terrible assumption about the word, same mistake adblock made and had to whitelist sites for. See if you can visit this URL as a test –  Nick Craver Dec 26 '12 at 21:49
@NickCraver I checked the site and it shows "('success');" (without the quotes) in the body.. –  Lizz Dec 26 '12 at 21:51
@Lizz - in all 3 browsers? I would bet some browsers are respecting your proxy settings and IE has different proxy settings. –  Nick Craver Dec 26 '12 at 21:52
@NickCraver all 3 use the same proxy and have the same proxy settings. FWIW, our proxy cert is self-signed but SO works beautifully in IE –  Lizz Dec 26 '12 at 21:54
But did you make sure all can load that URL? And does IE pass the automated test as well? (Maybe, if it doesn't pass, some other mechanism is used, which in the end makes it work for IE but not for the browsers that pass the test?) –  Arjan Dec 27 '12 at 11:32
@Arjan, all 3 passed, though IE8 wanted to save the file 'ad-block-check'; opening it in Notepad displays the familiar 'success' message, as with the others. I don't know the significance of this variant response, though I suspect it isn't much. –  Lizz Dec 27 '12 at 19:48
Ever solved this? –  Arjan Feb 25 '13 at 21:34
Nope. IE still working at work but not any FF or Chrome. :( Anything changed or new I can try? –  Lizz Feb 26 '13 at 3:16