Unless you're planning to do something really useful with the middle button other than the same thing you can achieve clicking the [ + ] button (new tab) at the "top bar" or using the the Ctrl+t shortcut, I would recommend to just pass the event to the parent.
For me this action is really useful given that I've it linked to -Lower- which is an action provided by Gnome (desktop environment) that puts the window behind. I don't need to switch workspaces, nor use keyboard shortcuts to change focus.
Considering that when I use the browser I usually have one of my hands in the mouse. The only excuse I would have for the need of a fast/handy new-tab-shortcut linked to the middle-mouse-button would be "to hide" rapidly the content I'm looking at. That is not the case, I insist, it would be much more valuable (and better for my freedom) if you give me back control of the window and let me decide how windows (in general) should behave.
This means, I don't want to search hundreds of properties at about:config. Let alone having to create an account to make a comment (almost a complain) about what is expected from a browser created by a huge community of people with common sense. Please just don't hide what is what, that is not the top bar of the window, it's the one of the browser (the app), it is Firefox's top bar. I understand that having the top bar of the window will do it ugly, but it is mine. This is not a bug it's a takeover of functionality.
We could agree about how useless the top bar of the window is, sure. But I don't think it is right to override an expected behavior unless I agree to do it, like in settings or installing some extension that change the properties/behavior of the window. Historically windows have been boring (that explains the Themes section) but think about the future.
Thank you for the amazing work here and the mobile version