These are add-ons that I like to install on the kids' PC.
Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
Collector's Note There's no reason whatsoever for kids to be subject to web advertisements. None. For starters, kids can't tell the difference between relatively harmless ads and ads that will install spyware if you give them a chance.
Direct access to your most visited websites
Collector's Note Thumbnail screenshots of the sites your kids visit give them a nice alternative to bookmarks.
Personas are free, easy-to-install "skins" for Firefox. Personas Plus extends that built-in functionality to give you even more control and easier access to new, popular, and your own favorite Personas.
Collector's Note Note: This is now bloated into Firefox, so the add-on is no longer necessary. You have it in Firefox whether you need it or not. My kids really get a kick out of changing the look of the web browser, and they can mouse over to preview. You might want to bookmark the Personas gallery for them.
Note from Mozilla: This add-on has been discontinued. Try Add-on Update Checker instead.
Notifies you when updates are available for your extensions and themes.
Collector's Note This can be configured to automatically install add-on updates so that small children won't get confused by the install prompts and get on with their web browsing.