I just assumed Java was out of date (again) and was surprised to see it still blacklisted after updating to latest version.
There's no part of the UI saying "We've permanently blocked all of Java by default". Even if you agree with the developer's ideological stance here (which you very well may not), the UX part of the job is completely botched.
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.
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.