Here is an example of a hack to trigger a macro whenever its macro group is activated. It uses the periodic trigger being set to its shortest interval, which is 1 second. 
Of course, we don't want the macro to be triggered more than once, so it ends with an action to disable its macro group. Right before that, it enables the complementary macro group – which can re-enable the first group.
As I said, this is a hack… but the example works.
If anyone wants to experiment along these lines: heed the warnings on the Wiki page about setting a short interval for the periodic trigger.
The first group is "Firefox tab == YouTube".
The group contains this macro:
The second macro group is "Firefox tab != YouTube":
It contains this macro:
