I'm guessing the intention here was that popup windows wouldn't have the maximize button enabled. This is not a web extensions bug, but how the Firefox front-end team decided is popups should behave. The only thing we can do from the extensions side is preventing you from maximizing popup windows using code, to avoid the inconsistency, which is probably the opposite of what you would want. In any case, I'm moving this to the Firefox component for them to decide if this is something they would want to address (or even if they would like us to actually prevent extensions from being able to maximize popup windows, for consistency).
Bug 1756507 Comment 5 Edit History
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I'm guessing the intention here was that popup windows wouldn't have the maximize button enabled. This is not a web extensions bug, but how the Firefox front-end team decided popups should behave. The only thing we can do from the extensions side is prevent you from maximizing popup windows using code, to avoid the inconsistency, which is probably the opposite of what you would want. In any case, I'm moving this to the Firefox component for them to decide if this is something they would want to address (or even if they would like us to actually prevent extensions from being able to maximize popup windows, for consistency).