See bug 434718, especially its = 6 to see the difference: the two box cascades behave differently in Firefox 2, because the em-based font-size "bounces" off the minimum font size).
These files are intended to be accessible by web content.
All other chrome content is now blocked from access by the web.
There is, however, a way for extensions to make their content web-accessible.
There are a number of changes in the upcoming Firefox 3 that may affect your web site or web application, as well as new features you may wish to take advantage of.
This article will serve as a starting point as you work on updating your content to take the fullest possible advantage of Firefox 3. Firefox doesn't currently enforce this rule (it did for a while during the development of Firefox 3, but too many sites break when this rule is enforced).