[Thanks to Jim Mathies, Firefox developer, for writing this post - with minor edits]
Firefox 3 will have some support for Parental Controls on Vista (see bug 355554). The download manager is now aware of situations where content gets blocked by proxies. Downloads that are blocked display correct UI message to indicate what happened.
A good example would be a right-click Save As on an image link that’s offensive. Something like that might get blocked by a proxy, so we handle that case correctly in the download manager.
As for the main browser area, inline display of offensive content tends to be handled by the proxies themselves, so that content downloaded by the browser will get replaced by some sort of “blocked message” content, or it’ll simply be displayed as a broken image. In general, we rely on proxy blocking in the main window right now. This is something we want to eliminate down the road by detecting blocked content before we issue the request and display our own message.
In preparation for that and for more general use, we now have an interface (nsIParentalControlsService) that interacts with the operating systems parental controls services, so extension developers and XULRunner apps can respect local parental controls restrictions.
Currently, we support querying of application level flags like file blocking, logging of blocked content, and requesting overrides for a single or list of URIs. We don’t yet support for querying whether a particular URI will be blocked, but we’ll be adding that soon. Until then, developers can rely on HTTP status results to detect blocked downloads.
Looking forward to Firefox 3.x, we’ll have better support for inline blocked content, and we’ll be adding checks of file blocking flags in other places, like the extension manager. We weren’t able to get that in this release because it required UI changes and parental control support was added pretty late in the release cycle.