Strange word wrap issue in 3.0’s Visual-mode post screen

I recently upgraded my 2.9.2 WP install to 3.0 with basically no issues at all, which was wonderful. However now when I attempt to write a post in the Visual-mode post screen, I get a strange 90-character word wrap going on which looks odd on my 1680×1050 screen. Here’s a screenshot of it: https://imgur.com/MWOQ3.png

HTML-mode is unaffected by this. Any thoughts?

Addendum: my browser by default is Firefox 3.6, but after this began I also tested it in other browsers I never use – current versions of Internet Explorer and Chrome, with the same result.