Add an option to dynamically show/hide the tab bar for better foldable support

[knowdark]

I have a foldable device, and find that generally Firefox Mobile supports foldables very well, barring minor issues. But there is one thing I'd wish it did better - the tab bar behaviour. I love having the tab bar when the device is unfolded! But I equally hate having it when it's folded: it just takes up too much space, isn't very useful due to limited space, and you can't have a bottom address bar with it, which I prefer, which also means I can't change tabs by swiping the bottom of my screen.

By adding the option for the tab bar to dynamically appear only when the device is unfolded (or alternatively when the screen width is high enough) we'd get the best of both worlds! 

[Jon]

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