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Also, you can use the tweaker option to show desktop by double-clicking or middle-clicking the taskbar. You can also use the "Reserve empty space on taskbar" option to always have a space to double/middle click. Refer to the help file for more information.

What kind of lag?

Regarding the vertical lines, you can lock the taskbar by right-clicking on it and choosing the Lock option.

Thank you for the support, I appreciate it!

Right, makes sense.
Combining options weren't designed to work with the "Combine when taskbar is full" configuration.

With "Combine when taskbar is full", Windows decides when to combine/decombine groups, according to the available free space, and so the tweaker options for combining have no effect.

I can think of only one explanation: perhaps you changed Firefox to specifically not combine.

  1. If you right click on Firefox's AppId (here), are there any configured settings?
  2. If you open multiple windows of a different app (say, Notepad), do you still see multiple items? 

That's strange, with "Combine grouped buttons", there should be only one Firefox item, not three. Can you open Taskbar Inspector when there are several open Firefox windows open, and post here the screenshot?

Why did you uncheck "Don't group pinned items"? Sorry for the misleading picture, I meant to refer only to the highlighted options. Please recheck "Don't group pinned items" and see if that's what you're looking for.