I had this issue with sponsored websites. It took me a while to figure it out, but this resolved it for me:

When you just disable showing sponsors, Firefox will still autocomplete them.

• You have to let FF show those sponsors on the homescreen.
• Then remove them one by one from the homescreen.
• After all are removed, you can again disable the sponsors in the settings.

If this guide is too vague, I can be more explicit.

Well, there is a simple solution to stop contributing to unnecessary animal suffering. Becoming vegan will not prevent this from happening tomorrow. But at least you (if you are not yet vegan) are no longer responsible for it.