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.
Stardock Product Lifecycle Manager