"Use system proxy settings"? Since WHEN?

Since Firefox 3.5 apparently... damn shame I don't find out about this until my work week is over!

My situation: I use Portable Firefox on a portable hard drive at work, and it's set to use the corporate proxy. I just nicked the settings from IE (well, the system, but I got 'em from IE) and bam, my Firefox works on the network. Nothing sneaky, just a required step to get another web browser online at work. Every now and then, I need to use that Firefox profile at home (as opposed to the Portable Firefox on a secondary internal hard drive at home), and of course Firefox goes nuts because it sends a request to the corporate proxy and, naturally, gets denied. (I hope they don't think I'm trying to hack them.)

Such a situation arose today, so I went out looking for proxy add-ons. I found one that looked promising... but it said it wasn't being maintained anymore because, since Firefox 3.5, you could just tell it to use the system proxy automatically. Wow, wish I'd been informed! Or perhaps, to be fair, wish I'd paid attention.

I'll have to see if this works Friday when I go back... I'm not entirely sure it will, since Chrome and Opera make the same claim (that they use the system proxy settings), and... they don't. Or at least they don't to the satisfaction of the corporate proxy. Every time I try to go to a page, it asks for a username and password, and the creds I use to log on to that terminal don't work. (Neither does just hitting enter.)