Select a URL in any app (including another Mozilla window), then click
the middle mouse button in the content area of the browser (but not
over a link, since that opens the link in a new tab or window).
Voila! No need to clear the urlbar or use copy/paste key bindings.
You can't run more than one instance of Mozilla at a time for a
particular profile name, because they might end up corrupting important
files like bookmarks.html or prefs.js. So what do you do if you
want to get the current running instance of Mozilla to show a page when
you click on it in another app, like xchat?
That's what mozilla-remote is for. From any app, call:
mozilla -remote 'openURL(%s)'
mozilla -remote 'openURL(%s,new-tab)'
mozilla -remote 'openURL(%s,new-window)'