M4 appears to use the browser font for the graphical widgets and for the JS/XUL dialogs. This looks very ugly. Mozilla should use the system settings for fonts (and colours) on all "non-content" parts of the UI, for example dialog boxes, command histories and text on widgets. Perhaps this could be done by auto-generating a CSS file from the system settings at startup and using those values when rendering widgets and dialogs. Later on, the mechanism could be extended so that the CSS is refreshed and the display is updated when the system settings are changed while Mozilla is running.

reassigning to hyatt as p4 for m7. Dave, should this be part of the font component or related to the system colors work in CSS?

CSS2 will have support for specifying native system fonts. A duplicate of this bug has already been filed against Peter Linss. Closing this one out.