I think what you want is in Firefox nightly right now: https://blog.nightly.mozilla.org/2024/08/07/firefox-sidebar-and-vertical-tabs-try-them-out-in-nightly-firefox-labs-131/
That expands and compacts based on the sidebar state and can be flipped to the right side of the window in the ‘customise sidebar’ settings.
Why not an inflatable airlock? It worked* for Voskhod.
*
*I mean it didn’t kill the crew…