First if all, try to hit alt+F3 and write firefox. You will see all qubes in which you can run it.
Second, this has been discussed a few times already.
Third, there is already an new App Menu designed by @ninavizz available for testing: New Qubes application menu. It has the possibility to present by apps and by qubes (and much more). See also: When Will Qubes Have The new App Menu?.