Thanks, I think I see the issue and shouldn't be hard to fix.
Bug 1947526 Comment 2 Edit History
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Thanks, I think I see the issue and shouldn't be hard to fix. This [call to `handleVerticalTabsPrefChange` from SidebarManager's init](https://searchfox.org/mozilla-central/rev/f89fa2093be1cedd708fb8c5538df98ba73f4456/browser/components/sidebar/SidebarManager.sys.mjs#81) is going to unconditionally set the visibility pref back to its default at startup, regardless of what the user might have customized it to be.