I suggest using your hosts file and an add-on called uBlock Origin ( https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/ ). I am not sure how effective those measures are. I have employed them for a particular site but I still expect that I have to be on my guard if I see any links to that site (such as in search results). On searching, I find a reference to this add-on: https://addons.mozilla.org/en-US/firefox/addon/blocksite/ and an explanation about the hosts file in this answer: https://support.mozilla.org/en-US/questions/848199

Thanks for this. I installed ublock. Now how do I get it to block the sites I want blocked?

As I said, no guarantee. Open add-ons (ctl+shft+a), the left panel should show the pane as 'Extensions'. Find uBlock Origin in the list, click the three dots at the extreme right and select 'Preferences'; that page will open in a new tab. On the "My Filters" tab enter the hostname you want to block. uBlock Origin does a lot more. You could spend some time looking at the tabs. If all you want to do is block that site, the other add-on may suit you better.