This add-on only sets safe search mode for search engines. You should not have received a link to xhamster etc by google, duckduckgo, etc. To block e.g. xhamster, you could use https://addons.vaultpit.io/en-US/firefox/addon/adultblacklist/ , which blocks known adult domains. In the case you posted above, https://addons.vaultpit.io/en-US/firefox/addon/jsguardian/ also worked (it checks the page text). You achieve complimentary effects by using these together.