I have observed some Firefox users for many years dislike the extension unified button. Please let us move the new extensions button into the overflow menu. I also understand such button is necessary to absorb Chrome users with similar UI.
It's a good idea to combine the two menus into one, just like what the prototype image (as attachment) shows.
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The whole menu is split into two parts. The upper part is the traditional overflow menu, where both extension browser-action button and Firefox built-in button can be arranged. The lower part is the extension list, just like that in Chrome, where all extension browser buttons are listed.
When customizing toolbar, the extension button in the lower part can be dragged into the upper part, and the built-in buttons in the left area can also be dragged into the upper part. Both of them can be dragged into the topside toolbar.
The lower part cannot be collapsed when the upper part leaves empty.
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The Chrome users would like leave the upper part empty to experience the truly "extension unified menu" as open-box experience. The advanced users can customized their upper part and optionally collapse the lower part to experience the footloose UI layout.
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