Since today I cant connect to any Website using a browser like firefox.
Browsers that are based on chromium work for some reason.
It works fine on my windows computer.
Firefox just tries to load a website and just stops to load after 30 seconds and Mullvad Browser just cant connect.
What can I do to fix this?
For anyone who has the same problem: You can fix it by enabling IPv6 in Mullvad VPN or disabling IPv6 on macOS entirely.
Or you can just use a Chromium based Browser
I suggest checking your proxy settings.
Have you used Mullvad Proxy with the extension? If so, can you try connect and then disconnect to a random proxy server?
Perfect, this worked for me!
Thank you, it worked like a charm 
Perfect!!! thanks so much. I already doubted using firefox. Fuck chrome!
you are the man, this worked perfectly for me
Just ran into this myself, and this also solved it for me. Thank you!
So I disabled all proxys in Firefox and in the Mullvad Browser, I still cant connect to any websites.
Disabling the Mullvad DNS also didnt help.