In the Desktop & Dock settings on my mac i have set `Double-Click a window's title bar` to `Fill`, and it works not only with native applications, but others as well, for example Discord, Chrome etc. I am not a macOS developer so I don't know if there are any obstacles that prevent this feature from being added, but i use it very often, and it is a bit annoying that Firefox doesn't support it. For reference I am using an Apple Sillicon MBP with Sequoia 15.6.1, and Firefox 15.6.1. Note: I know there is a keyboard shortcut for doing this, but I am used to double clicking the title bar when adjusting my windows across the screen.
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