This is how I add it. I wasn't aware there was any other way. I open that panel, drag the tool on to the toolbar, and Close the Customize Tools panel.
I was using version 2.1.0 on Windows, which I updated to 2.1.1 yesterday.
This is a good question. I don't think I actually ever specifically closed it--most likely, it ended up being closed via global restart several times. And that might have been the reason the tool didn't stick, because I tried closing and reopening 2.1.1 and I seem to have the Fill Tool on the bar!
Will report back if something goes wrong again.
Just to be complete--no, I'm on Windoze. Thanks though!
Thanks for the replies, all.