From MozillaZine Knowledge Base
Firefox and SeaMonkey do not feature built-in parental controls, content filtering, pornography blocking, or website blocking features. Thanks to the extension mechanism, however, you can add these yourself. Additionally, web filtering software is available which will filter the internet for any program on your computer.
Extensions can provide content blocking but they can be easily disabled by a knowledgeable user and cannot provide filtering for other software, such as Internet Explorer or instant messaging applications.
There are several Greasemonkey scripts which can be used to filter out vulgar language. As with other extensions, Greasemonkey is easily disabled and the filters cannot affect other programs.
Web filtering software
Third party web filtering software can filter objectionable content from all programs. Many Internet Service Providers provide free parental control mechanisms. Contact your ISP for more information.
While no software can completely block all objectionable contact, third-party software exists that can provide a very high filter level. Some examples:
There are also a few sites which review the effectiveness of such filters:
The most reliable filtering of content is done before it gets to your computer. If you have a broadband internet connection, then you can use a router with parental controls. This router would be placed between your cable/dsl modem and your computer. If you have multiple computers, they can all use the same router, so you need not purchase multiple copies. You may optionally purchase a 3rd party filtering service which the router can use.
Routers with parental filtering:
Do a search on -- "Parental controls" site:example.com
Related bug reports