Mullvad’s VPN software has an available function that blocks network traffic when the VPN isn’t connected, so there’s no need to patch that into the browser.
Does it offer the same system-wide protection as the desktop VPN product; or, does it only use the VPN for socks-proxied traffic through the extension-created SOCKS port, and so those protections are applied within the browser; or, it doesn’t protect against temporary interruptions; or, orher?
I can’t experiment with this during my workday, and we’ve reached the limit of information available without running it and testing, so I can’t help resolve this further right now.