Tested in both Firefox Greasemonkey and Chromium. Should work on any page where comments are wrapped in "div.outercomm".
To install in Chromium, open chrome://extensions/ and then dragdrop into the list.
To install in Greasemonkey, just click the link.
I could have made it do a fancy slide animation but it was kind of glitchy so instead of fixing the bug I just made them disappear instantly, thus concealing the problem. I also could have made the [-] change into a [+] when collapsed, but I'm a lazy fucker. At a total of 39 lines including comments you can tell I really worked hard on it.