Problem:

Firefox has suddenly stopped opening some websites.

Solution:

1. Type “about:config” (without quotes) in the address bar.
2. Click the “Accept the Risks and Continue” button.
3. Search for “network.trr.mode” (without quotes).
4. Click the edit icon and set the value to 3. This forces Firefox to use DNS over HTTPS.
5. Save the change and try opening the webistes again.