This issue began appearing in FileMaker 14. It was reported in 14 and 15, and intermittently seems to have been fixed but it keeps coming back like a bad penny. Or worse than a bad penny, maybe a bad nickel that is 5 times worse.
I don't want to work with the standard toolbar icons when I am doing development work. I don't want to accidentally create or delete records. All I want to see is Manage, Data Viewer, and Debugger.
Every time I use Customize Status Toolbar, I get the satisfaction I want for a few days and then, without any warning the default set is back again.
And it only happens in Browse Mode. The customized tool bar I made for Preview Mode and Layout Mode do not vanish.