You know those big, yellow bars that browser and websites tends to come up with when they want to be 100% certain that the user notices something is up?
I've yet to see a single non-technical user even notice or react to its presence once. I see it instantly and can't understand why it doesn't alert or annoy them, but to them, it's just not there.
In light of that sort of behaviour, adding a subtle icon to the location bar is meaningless. Heck, adding anything to the location bar is meaningless if the intent is to communicate with the user; most users never look there.
So yeah. If that's Mozilla's stance, they will find out that nobody's going to notice. I certainly didn't see it. That is effectively dead code which they've written.
Pressing on that icon should allow you to run java once, or allow if forever for that site.
If the icon doesn't appear, I think you should file a bug on bugzilla.