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Mozilla is testing new tab wallpapers in desktop Nightly
(i.imgur.com)
A place to discuss the news and latest developments on the open-source browser Firefox
Dang, it's not a quick setting in the UI, and using an extension didn't seem to work properly.
Maybe I'll raise that as a feature request, I think others may prefer that to having the separate "view" tab (ctrl/cmd + T is pretty convenient).