Works fine on http://feeds.macrumors.com/MacRumors-All - what URL doesn't work?

EDIT: user found out that the reason for this was that they had Firefox's standard "HTTPS-Only" mode on and that MacRumous feed supports just HTTP. Even disabling the requirement for the site doesn't help because apparently HTTPS requirement is still enforced for every URL using XMLHttpRequest API that Feedbro uses.

Bug report has been submitted: https://bugzilla.mozilla.org/show_bug.cgi?id=1692811