Great Addon, have been using it for years and am very glad that it works with FF57 now.
I only have two minor complaints:
1) In some recent version the keyboard shortcut for opening Brief changed from Ctrl+Alt+D to Ctrl+Shift+D. Unfortunately the latter shortcut is already used by firefox for bookmarking all tabs. Thus Brief cannot be opened anymore using a keybaord shortcut.

2) Yesterday Brief suddenly deleted all my feeds and I had to resuscribe to all of them. Didn't happen before and I hope it won't happen again.

Anyway, great addon, thanks a lot!