Is there any way to reduce more than the minimum width that browsers allow

You're thinking wrong.

You're thinking "How can I modify existing software to better use a widescreen monitor", when really you should be thinking How can I now use my computer more efficiently. Stop maximizing. Maximizing stopped being useful about 5 years ago*, we have screen resolutions in the 1000s of pixels both ways now, and enough hardware to run countless programs at the same time.

Start using multiple applications at the same time! Wheras before you might have a document on the secondary screen to refer to while you work on the primary, have both on one, and then use the secondary for either even more reference material, or something completely different (I keep IRC channels always-open, cover them up with the docs to whatever language I'm using at the moment, so I can look up functions and arcane chants very quickly).

On windows 7, use Aero Snap, on Vista and below use software such as GridMove or a suitable Autohotkey script (It's really not my area, but AHK serves my needs brilliantly)

Buy a 5 button mouse, you're spending lots on your output, so you should beef up your input. Autohotkey, or software such as Logitech's SetPoint, can empower additional buttons, or combinations, to help you with your workflow.

Perhaps try sidebar software, such as Desktop Sidebar, Google Desktop, or Vista/7's built in sidebar. Join Twitter, and keep a client always-open to keep track of news and current events. Basically, a widescreen monitor, in the right hands, is a wonderful thing.

2 is computing paradise.

*at least, programs stopped being easier to use maximised. It's still great for those times you just need a little extra space.

Originally I posted the extension Yarip as being able to accomplish what you want. As pointed out by JohnKiller in many cases it won't do the job because of the width/margin issues that he mentioned.

Firebug + Firediff (to export the changes) + Stylish (to make changes persistent) is definitely the way to go. I was able to identify what was needed to get rid of the sidebar and reflow text how you wanted in a very short time.

I've included a stylish style snippet to take care of the sidebar just so you can see how straightforward it is. All of the values to be changed were found with firebug.

@-moz-document domain(superuser.com), domain(stackoverflow.com) {
    #sidebar, .sidebar {
        visibility: hidden
    }
    #mainbar, .mainbar {
        width: 980px
    }
    .question-summary {
        width: 980px
    }
    .narrow .summary {
        width: 799px
    }
}

Note that this works on the main page only. You will add a few more changes to get the Q & A pages to look right.

That said, if you really don't want to delve into the CSS (Firebug makes this really easy) there is another extension that you can try: HackTheWeb. This allows you to both remove divs and expand and reflow others to compensate with just a few keystrokes, no modifying CSS required. You will still need to install the extension Stylish if you want to save your changes. Take note of the version restrictions (as noted on the HackTheWeb page) required to get your changes to persist.

Honestly though I think you will spend alot of time fiddling with it trying to get it to accomplish what you want because it just doesn't have the power to modify all properties like Firebug does. I tried - it definitely doesn't work that well on Stack Overflow without diving in and manually writing rules in Stylish.

If you do choose to use it though, here is a guide to the keystrokes.