Currently, if the user opens the new Migration Wizard, if the user doesn't have a pre-existing about:preferences tab, we'll open it at the end of the tab strip. For a user with many tabs, that can be somewhat disorienting - especially if the user opened the Migration Wizard via something like the logins autocomplete popup.

It looks like we have the ability to open tabs as being "related to" the current tab here: https://searchfox.org/mozilla-central/rev/736eb58cd30da3afc0310b58232a1e4d0dcc86a4/browser/modules/URILoadingHelper.sys.mjs#329-330

Unfortunately, the switchToTabHavingURI call we're using to open preferences doesn't pass that value. It'd be excellent if openPreferences could take the relatedToCurrent as an extra argument, and pass that along to switchToTabHavingURI (I believe switchToTabHavingURI will already plumb any extra arguments passed to it to the tab opening routine if a new tab is being opened).