To save some searching, "Implement 'Fit to window width' as in Opera 8.0 (no-horizontal-scrollbar mode)" is Bug 276166 in Mozilla Bugzilla. In a little more than four years, it has accumulated twenty votes, ten comments, and no developers. Still, upstream is the place to fight for this feature. Vote the bug up. The going in-favor argument there seems to be tied to small screens on UMPCs and netbooks. As that's a new and fast growing market segment, you might find some increased interest in the feature.