I’ve installed uBlock Origin on Firefox for Android (version 10 in my case) and I’ve noticed that advertisements are blocked, but the relative empty spaces are not removed as they are with the same sites using Firefox desktop.
The option “Hide placeholders of blocked elements” is enabled.
I forgot to point out that the Android device is a smartphone with the 6.5-inch screen, and consequently the version of the site that is loaded is specifically for these devices and therefore different than the one for desktop PCs …