Full-screen Safari is nice, but some pages don't do well with the Reader option, and are hard to read on an extremely wide-screen monitor.
If you move the mouse cursor all the way left (or right), the pointer changes to a horizontal-resize indicator. At this point, you can click and drag toward the center, to change the width of the page while the browser stays in full-screen mode. A gray linen background appears on the left and right of the resulting narrower page.
Each tab can be adjusted independently.
[crarko adds: I tested this, and it works as described. I tried this in Preview and it seemed to resize the page automatically when switched to full-screen mode.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110722135213505