How do I change Firefox's user agent via about:config?

My work has blocked Internet access with Firefox. I know how to change the value of “general.useragent.extra.firefox” through about:config, but when I try to change it to IE, I still can’t access any website.

I have Firefox 3.5.6 and Internet Explorer 7 installed on my desktop. Now, would I have to use the user agent that Internet Explorer 7 is currently using on my computer or can I use any Internet Explorer user agent? The user agent that Internet Explorer 7 is using right now is:

Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; InfoPath.2; .NET CLR 3.0.04506.30; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022).

Do I just copy that and use it as the user agent in Firefox?

How can I make this work?

That setting influences only a part of useragent string. Try the general.useragent.override one. This is what User Agent Switcher extension does. Tip: this setting may be not present in fresh install. You can create it with context menu.

Also, if you think you cannot install Firefox addons when it's not allowed to browse Internet, here's another tip: just go to Firefox's addon site site and download XPI extension archive; the just open it in Firefox with File -> Open menu and it will install!

Point your browser to about:config, right click, New -> String

Enter general.useragent.override and then

Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; InfoPath.2; .NET CLR 3.0.04506.30; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022).

If your Firefox is completely blocked by the firewall, you should try some other Firefox-based browser like Flock. Flock can use any add-on posted on Mozilla add-ons homepage.