You may be interested in trying our Developer Build! It is still in its early stages, but it will give you a nice preview of what type of features you can expect in Brave 1.0 - including extension support!
If you’d rather stick with our Muon build, this is likely an issue with the Strict Site Isolation security feature. Disabling this should resolve the issue. Go to: Settings --> Security --> Strict Site Isolation and toggle the option to “off”.
If this doesn’t resolve the issue, let me know and I’d be happy to continue troubleshooting with you.