I had this problem as well and managed to fix it by uninstalling the 'unclutter' package. This program hides your mouse pointer after a couple seconds of it not moving, which apparently also triggered the Firefox tooltip to disappear. I uninstalled this package, logged out and back in, and now my Firefox tooltips work again.
I had this problem as well and managed to fix it by uninstalling the 'unclutter' package. This program hides your mouse pointer after a couple seconds of it not moving, which apparently also triggered the Firefox tooltip to disappear. I uninstalled this package, logged out and back in, and now my Firefox tooltips work again.