Is in that a fitting internet filter (parental control software) that works on firefox and explorer? (cheap?)?
Answers: www.k9webprotection.com
It is completely free and will filter firefox and IE.
I'd have to utter do not bother if your child is 13+. All your kids have to do is google the describe of the software in interrogate and they will find ways to get around them.
If they are younger you could probably try using the parental control features already present surrounded by Internet explorer and firefox. Or some ISPs like Rogers proposition free parental control software but I have no conception how good it is.
You can also check out this register of user reviews on parental controls to help you agree on:
http://www.monitoringsoftwarereviews.org...
Try this one.. it is one of the cheapest parental controls you can get..from parents software.
http://infolib.qckbuy.com/parentalcontro...