Unfortunately, uMatrix (the other gorhill project) has bad bugs on Firefox.

When you navigate to another page, it will sometimes get the host for a web request wrong, causing it to apply the wrong rules for that request. This can cause you to lose your login cookies. And since uMatrix has been discontinued, so there's no more central repository to collect bug fixes.

A fork is available called nuTensor which does resolve the cookie clearing problem, but I often see hosts that are not part of the current page (they were from the page I was previously viewing) appearing on the grid.