I have no idea what mailspring has or had. Thunderbird however has two "combined" inboxes and has done for many years. (over 100 version numbers in fact) Initially it has a global inbox which incorporated all mail for POP account into a single global inbox under the local folders account. See https://support.mozilla.org/en-US/kb/unify-your-pop-email-accounts-global-inbox Later, in the Thunderbird version 3 timeframe a unified inbox came along. which is discussed on the products home page https://www.thunderbird.net/en-US/ There is support documentation on the unified inbox here https://support.mozilla.org/en-US/kb/switching-folder-pane-view

Thenk you Matt! I am using the unified inbox and I am a happy camper.