I agree with the sentiment that preventing autocomplete is stupid behavior. I find it mildly offensive that the browser enforces this, without option to turn it off, since it is supposed to be acting on my behalf. "Fix it yourself" is generally not a very helpful answer. However, in this case, I eventually did fix it myself (after I read how).
There are bookmarklets floating around which will force autocomplete for a page, but you have to load the page, then hit the bookmarklet, and it's not (that I've seen) a 100% solution. Better than nothing, though, and it works as a non-admin user.
Ultimately, to remove this behavior, I ended up altering a system file. I have to edit it again every time I upgrade Firefox, but that's part of the documentation for my system, now. On Gentoo, running FF 3.6.20, the file is /usr/lib/xulrunner-1.9.2/components/nsLoginManager.js. There is a function named "_isAutocompleteDisabled". Alter it to unconditionally return false, and the effect is that autocomplete is never disabled.
That's just my system, and I obviously use a pretty old version of Firefox. If you figure out where the current version of Firefox keeps the equivalent files on your OS of choice, and grep around for "isAutocompleteDisabled", I think you'll likely find the right place to hack the newest versions.
I am very pleased with the results. Autocomplete is no longer conditionaly, and I am never bothered by a site's attempt to prevent it. On the other hand, I'd be even more pleased to find a solution which is as effective, but stays within the confines of "normal" user configuration - an extension, or greasemonkey script, what-have-you.