In IE 6 for the PC, go to Tools->Internet Options, view the General tab, and click on the Accessibility... button. Here is where the user can specify a default stylesheet for IE. I've looked around Mozilla's Appearance dialog, and all it has are font and colour settings, rather than a user stylesheet setting for truly cascading stylesheets.

what Do you mean with : default stylesheet ?

I mean a user-specified default stylesheet that will be applied to the formatting of all documents loaded in, so that the user has fine-grained control of what they see (As opposed to some chrome stylesheet for toolbars.) Font and colour just isn't enough. Go play with the feature I describe that is in IE5 or IE6...

Looks like a dup of RFEs like bug 41975, bug 45848, bug 68416 (see also tracker bug 107023) *** This bug has been marked as a duplicate of 45848 ***