You could consider an add-on :

https://addons.mozilla.org/en-US/firefox/addon/foxfilter/

https://addons.mozilla.org/en-US/firefox/addon/adult-blocker/

https://addons.mozilla.org/en-us/firefox/addon/cloudacl-anti-porn-pro/

Note that starting Firefox in Safe Mode would disable add-ons.

Search the web for child protection programs. Many are free.

Here's an example of a filter:

http://www1.k9webprotection.com/

It has a lot of features, so hopefully you can set it to block just what you want blocked without additional difficulties.

FredMcD said

Note that starting Firefox would disable add-ons.

Sorry, Starting Firefox in Safe Mode would disable add-ons.