Il love this extension, thanks for making it! Unfortunately it doesn't work with FF 57 for me either, Firefox says it's a legacy extension. Screenshot: https://elzevir.fr/tmp/brief.png Using brief 2.4.0 with Firefox 57.0 on Linux (Arch, Firefox installed from the Arch repo).
I guess it must be some small detail, since the release notes for 2.4.0 say it's the WebExtensions edition. Please, please, please make it work! I really don't want to use another feed reader. Thanks!