Umatrix is in my opinion the best tool to stop third-party trackers and any resource loading from external sites during web browsing. Since the matrix overview can be used quite easily to unblock certain trackers I use the default setting of blocking every external page resource. This can lead to problems when a website uses popups, e.g. for third-party logins, since firefox only shows a reduced taskbar when opening a popup. Taskbar of a firefox window in popup mode
I did not find an easy way to show the umatrix icon in this popup mode (even the official documentation is more
of a workaround than an actual solution), but firefox allows opening popups in regular tabs, which works quite well.
Unfortunately this functionality is not exposed in the regular settings and can only be activated using
0 and all popups should be opening in new tabs (together with a working umatrix icon).