The Mozillazine Knowledge Base has an article Pane and Menu Fonts which describes how to use the userChrome.css file to fix font size problems in the "chrome" part of the Iceweasel user interface - that is, the menus, dialog boxes, and so on.
This version of Iceweasel is Iceweasel/2.0.0.17 (Debian-2.0.0.17-0etch1).
The following code, placed at the end of the userChrome.css file successfully fixed the problem of the Iceweasel menu fonts being unreadably small at 1600x1200 resolution:
/* Global UI Font */
* { font-size:16pt !important;
font-family:FreeSans !important;
}
