Well, then the folks at Tbird have the most bizarre release channel I've ever dealt with. I've been on release channel for years, with automatic updating, and if I go to the Help, About dialog (ALT+H,A), I watch the check being done and am told that Thunderbird is up to date. I am on 68.12.0 32-bit.
That's one heck of a primary version jump, from 68 to 78, so I can't explain it if it's not the beta channel, unless we're talking 64-bit, which the folks at Tbird used to recommend you not use, I believe. It gets confusing when you've had something on a given machine for a long period of time.
Brian - Windows 10 Pro, 64-Bit, Version 2004, Build 19041
A gentleman is one who never hurts anyone's feelings unintentionally.
~ Oscar Wilde