Even though the taskbar buttons is set to never combine
We need to know if that's happening in what you posted - meaning - you have multiple instances of Firefox open there?
I appear to have the same settings you have pictured and can't seen to reproduce the issue:

If it is reproducible, you likely have some other setting that I do not.
While you would have to configure Start11 again, it might prudent to purge \ reinstall and then try this as the very 1st settings change. If it works as expected, change other things to see what is causing the conflict.
Sean Drohan
Stardock Product Lifecycle Manager