There's got to be an about:config entry that tells Firefox whether it's already displayed that tab, which, at worst, you could manually edit before starting. B

There's got to be an about:config entry that tells Firefox whether it's already displayed that tab, which, at worst, you could manually edit before starting. But, even if that were an acceptable solution, I couldn't find it with a quick search for "display", "update", or "tab", nor even find an up-to-date list of entries (I think that http://kb.mozillazine.org/About:config_entries http://kb.mozillazine.org/About:config_entries is quite old). EDIT: Actually, I know for sure that there's some preference that controls this, because I just updated and didn't see the new page; but I forget what it is. One suggestion I saw in https://superuser.com/a/1392487 https://superuser.com/a/1392487 was to change various addresses to non-existent pages; based on the names of their default values, `app.update.url.manual` might be a relevant one.