Hi! I think you're misunderstanding what that TorBrowser option actually does -- it's not something that could coherently be added to Firefox.
That "content alignment" config option isn't a standalone feature -- it a *piece of a broader anti-fingerprinting feature* that's specific to TorBrowser, which they call "letterboxing". Basically, TorBrowser constrains the web content rendering area to be some "standard" size that's typically **smaller than your browser window**. This necessarily means you end up with some unused dead-space in your browser window, and so TorBrowser gives you this Content Alignment option to let you decide whether you want all that dead space at the end vs. evenly-spaced.
There's a blurb in the settings in TorBrowser to explain it a bit, which I think was just-off-the-top-of-the-scrolled-area in your screenshot -- that explanation says:
> Content alignment > Tor Browser's Letterboxing feature restricts websites to display at specific sizes, making it harder to single out users on the basis of their window or screen size. Learn more > Choose where you want to align the website’s content.
Firefox doesn't do this letterboxing thing, because we aim for a different place on the usability-vs.-fingerprintability spectrum. So Firefox doesn't have any such dead-space for content to be aligned into.
To illustrate Tor's feature a bit, here's how an extremely-plain website looks in TorBrowser with "content alignment: top" vs. "content-alignment: middle".
Top:
Middle (notice that now there's a strip of purple dead-space between the URL bar and the white web-content-rendering-area)
The purplish/bluish area around the side here is the "dead space" that I'm talking about, which TorBrowser is taking away from the web content rendering area (and then letting you choose whether to put the web content area top vs. center aligned in that dead space).
In Firefox, there's no such dead space; we just fill the browser window, and there's nothing to align.
