NetworkError when attempting to fetch resource.

[alant54]

Please complete the following checklist (by adding [x]):

• I have searched open and closed issues for duplicates
• This isn't a feature request
• This is not a report about my app not working as expected

---

Using Mullvad Browser with Protonvpn Firefox extension on Linux is not working anymore.
I already tried to reinstall the browser and the extension, but nothing works.
After installing the extension I get "NetworkError when attempting to fetch resource." with the button saying update if I click update I will be send to the Firefox add-ons page reinstall it but nothing changes.
On Firefox and Firefox-esr the extension is working.