Perhaps because they are still on very small monitor where horizontal space is precious, Mozilla is FLOSS but definitively not a free project, they act as a corporate one so maybe they have some guidelines written in the era of 1024x760 CRT monitor, I can't tell...
However I'm glad I can have them (hiding the tab-bar also to avoid clutter) since with them I can have many tabs open, seeing a sufficient amount of text to identify any and sparing precious vertical space on my modern normal 19:9 monitor...
I do not use Windows, thankfully, so I do not know Edge, I've looked for something similar for Chromium but finding nothing like you... Honestly I see modern WebVM improperly named browsers for legacy reasons as a necessary evil since unfortunately the modern world is web2.0-centric and I have not much choice for that, but I have not much expectations from both nor not much interests beside the minimum protection and ergonomic of my daily digital life...
However I'm glad I can have them (hiding the tab-bar also to avoid clutter) since with them I can have many tabs open, seeing a sufficient amount of text to identify any and sparing precious vertical space on my modern normal 19:9 monitor...
I do not use Windows, thankfully, so I do not know Edge, I've looked for something similar for Chromium but finding nothing like you... Honestly I see modern WebVM improperly named browsers for legacy reasons as a necessary evil since unfortunately the modern world is web2.0-centric and I have not much choice for that, but I have not much expectations from both nor not much interests beside the minimum protection and ergonomic of my daily digital life...