Link shorteners and redirectors, especially new and lesser-known ones tend to get caught in the fray with things like Google Safe Browsing (which FF uses as well) and Smart Screen.

It's because the original/shortened link gets reported and not the real/destination site. Then the domain (of the shortener/redirector) gets flagged, instead of the real site.

This happened to me at work this very week, with a redirector service that's a part of our email security stack. FF and Chrome were both blocking links that were safe, because the redirector service itself was classified as sus.