The Block Site add-on is not by Mozilla but a third party (there are a few with similar names). I remember seeing something about an add-on being installed without the users knowledge, and I'm fairly sure it was an add-on with a name like block site. I have been searching and can not find it so don't know the outcome.

Check your list of extensions, and uninstall it.

In order to uninstall a possibly unwanted extension, please do the following:

1. Click the menu button and choose Add-ons.
2. Once the Add-on Manager has opened in a new tab, click the Extensions button on the left side of the window.
3. You should now see a list of your installed extensions on the right side together with buttons on the right side of each extension.
4. To remove an extension from Firefox, simply click the Remove button. You should see a message that informs you about the successful removal of the add-on.
5. Note that some add-ons require a Firefox restart to be removed completely. To perform a Firefox restart after the add-on removal, click the Restart now link in the message.

You can find further information about uninstalling extensions in the following articles:

• Disable or remove Add-ons

• Remove a toolbar that has taken over your Firefox search or home page

I am having the same problem. This is no solution, because I installed BlockSite intentionally years ago. It has always served its purpose, and still does so. The problem only occurs since it has been (automatically) updated to the version that is compatible to Firefox 57.