Separate first launch homepage pages from new window launch if firefox is already running

[PlzMakeFeature]

I use a lot of pages as my homepage pages on firefox launch. However, whenever I open a new window, it reopens all those pages and adds a lot of unnecessary friction closing all but one to have a new window. It would be great to have a toggle option to separate new window startup pages to be able to choose what that shows (the same way what new tabs show is customizable) when firefox is already running instead of opening the homepage pages a second time. 
Thanks for reading!

[Jon]

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