Add the last Mullvad Browser Extension (0.8.4) to 13.0

MBE 0.8.4 is now live, see:

• https://github.com/mullvad/browser-extension/releases/tag/v0.8.4-firefox-beta
• https://cdn.mullvad.net/browser-extension/

This need to be added to MB 13.0, as it pins the extension icon to the navbar. (tor-browser#41581 (closed))