On my Mac running Sierra with Firefox 66 (and previous versions of FF) the tabcenter-redux tab title tooltips appear on top of not only web content in Firefox, but on top of other Windows when Firefox is not in focus.
So far the only way I've found to make the tooltips disappear is to hover over an empty area in the Firefox sidebar, but this isn't a very satisfying workaround.
On my Mac running Sierra with Firefox 66 (and previous versions of FF) the tabcenter-redux tab title tooltips appear on top of not only web content in Firefox, but on top of other Windows when Firefox is not in focus.
So far the only way I've found to make the tooltips disappear is to hover over an empty area in the Firefox sidebar, but this isn't a very satisfying workaround.