> Multiple browser windows seems redundant with multiple tabs or the Group Tabs mentioned above.
Surely it's the complete opposite: if you have multiple windows, then why do you need Group Tabs?
Yep! Hold right-click and use my scroll wheel from anywhere in the browser window. So not the userChrome (tab area, bookmarks bar) or off-screen (my OS desktop).
I have another program that is "tab aware" bound to a different hotkey: I can press alt+space and begin typing in the Title name of a tab to open that tab from truly anywhere on the screen. It could be nice to be able to have "global hotkeys" for a browser that does the same thing.
>Surely it's the complete opposite: if you have multiple windows, then why do you need Group Tabs?
One is passing off handling multiple groups to the Operating System (handle multiple instances of your browser) while the other is the browser handling multiple groups of tabs. Different ways of handling the same thing with trivially different workflows (alt-tabbing and hoping you alt-tab to the right instance of your browser or {Ctrl+Shift+E / Rebound Hotkey} for tab groups and selecting the proper group that you can name.
Being able to name my tab groups instead of having 19 different instances of "Firefox.exe" helps when trying to find the proper group rather than the proper window.
Multiple windows lets you split screen estate though! But IIRC there are add-ons to let you split the browser screen to render 2/3/4 tabs at once for that use case.
They have their pros/cons but have a lot of overlap and redundancy.