macOS 10.12.5 ignores standard browser

Hi there

since the update to macOS 10.12.5, some applications that come with a web interface (e.g. sabnzbd, also Sublime) always open their web interface in Firefox instead of Safari even though Safari is set as the default browser in macOS settings AND those applications are set to open using the system's standard browser. I tried changing the macOS standard browser back and forth, but they always open in Firefox.

Files that are associated with those applications (e.g. NZB files, being associated with sabnzbd) now of course open in Firefox too

This behavior is new since macOS 10.12.5 - until recently, sabnzbd (as an example) and its associated nzb files used to open in Safari just perfectly!

On the other hand, normal web links (e.g. links in e-mails, iMessage etc.) still open just fine in Safari as usual (as Safari is, like I said, set as the standard browser in my macOS).

The only workaround that helped was actually to remove Firefox from my system. Then, sabnzbd opened in Safari just like before. But as soon as I re-installed Firefox, sabnzbd is again opening in Firefox although Safari remained the standard browser.

Please help!

Gero