My fix was to find a single color FF theme which matches the current Windowblind and changes the text colors on the tabs.  If you hover over the various themes for FF, you can see the effect in real time on the tab text.

The problem is that once the text color has been turned to black nothing will change it. Changing themes, going to the default theme, uninstalling WB, uninstalling and re-installing FF, even a system restore back to before the WB update doesn't work. The only thing that fixes the problem is a system image restore. The problem isn't related to any WB theme. Just updating to the latest version somehow affects that one single FFox setting that governs tab text color, and it only affects the highlighted tab or a tab that's hovered over. It sounds like a small issue but with most themes it makes tab text unreadable.  Some bright themes will change the tab text color from white to black but it's not the same thing, and once it's been changed to black by the WB update it's stuck like that and no theme change has any effect. I can't imagine how a WB update could affect just one FFox style setting.

Doesn't matter what WB theme is applied, once the latest update is applied tabs go black and stay that way no matter what, even if WB is completely uninstalled or rolled back to an earlier version. Some setting gets changed that can't be restored without re-imaging the whole disk.

I think I made a mistake upgrading to the new WB10 when I'm still on Windows 7. It doesn't add anything but it does seem to cause some compatibility issues.