Watching a five year old is great way of getting feedback on how intuitive a UI is.
When you click FF in Budgie panel, it takes a while to come up and doesn't show any kind of "in progress". User (5yo attention span!) then clicks dozens of times and eventually gets dozens of FFs.
Some heavy apps appear to have their own run-once defense (e.g. LibreOffice) but not this one (and it's not really something to assume). It's also inconsistent with what multiple clicks do once the app is started.
Fully understand the issue here is with interpreting "app is started" from user perspective vs system perspective.