The mobile session store already does this (both in Fennec and in the current, simplified GeckoView implementation), but the desktop session store needs to handle that, too, once pinch zoom comes to desktop as well.

See also Bug 1511823?

FYI, I was talking with Kris today about bug 1660054. It seems like that bug may be a release blocker for dtz, depending on how frequently it gets hit by users, and fixing it may involve front-end work that may not fit into the 82 cycle. If that's the case, we might want to consider not fixing this bug, so that users can more easily reset their zoom by reloading the page or reopening the browser.

I realize that other browsers do save/restore the visual zoom but I don't know if we need to consider it as a release blocker for FF - in the general case the layout viewport that already gets restored should contain the visual viewport, so the save/restore scenario still puts the user back where they left off, just showing a larger content area. That seems like behaviour we could live with for a release or two if it takes that long to get some front-end support for resetting zoom.

Not actively working on this but we are tracking it as part of FFXP-664.