How many clicks on Unity launcher?

[Jim Byrnes]

Should an application run with one click or two on it's icon on the 
launcher?

I just installed 12.04 sometimes I can run a program with one click and 
other times it takes two.  Usually the first thing I open is Firefox 
followed by Thunderbird.  They require two clicks while the terminal 
icon I pinned to the launcher only needs one.  If I close and reopen 
Firefox one click will open it.

What are the rules regarding this?

Thanks,  Jim