Review Mozilla 1858801: Consider to bring back option to show a spinning cursor while the page is loa...

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1858801

Perhaps something we want to also bring back? browser.spin_cursor_while_busy will switch the mouse cursor to loading/spinning state when (focused?) page is loading.

/cc @henry @donuts @ruihildt

We should be able to just try out the pref in the current desktop alpha.