There is a "Report site issue..." entry in Firefox's hamburger menu, under "Help", which leads to <https://webcompat.com/issues/new https://webcompat.com/issues

There is a "Report site issue..." entry in Firefox's hamburger menu, under "Help", which leads to <https://webcompat.com/issues/new https://webcompat.com/issues/new>. This is really the best way to help Firefox with compatibility issues, i.e. providing specific information which can be acted upon.