I'm not having this issue. I always upgrade via Help > About Thunderbird

I'm currently on Thunderbird version 91.9.1 (64bit)

Do you have this setting in Thunderbird ?

• Menu app icon > Preferences > General
• Under 'System Integration' section
• Select checkbox 'Always check to see if Thunderbird is the default mail client on startup'

What Operating System are you using ? I'm using Windows 10. 'Default apps' has Thunderbird selected as default.

If Thunderbird not set as default, then what app is getting auto selected as default?

How are you discovering the -not set as default issue - what action are you performing when you discover it? Example: You have an image or document and you want to send it as attachment in email to someone. Right click on saved file and select 'Send to ' > 'Thunderbird Email'

I too and running the latest version of Thunderbird v91.9.1. and the Always check... setting is set. I am running Windows v8.1. Default apps goes back to Mail as the default app and changing those settings does not allow other apps; e.g., MS Word, Acrobat, nor Send To, do not offer the ability to send via Thunderbird. Changing those Windows settings does not fix the issue. The only way I have found to fix the issue is to download the latest version as the exe file and install it again and then the settings are correct. So again this issue has been occurring for months now that when installing Thunderbird via the Help function the default mail app does not retain Thunderbird settings in Windows.

Sounds like the OS does not recognise the TB update as the same TB installed set as default program.

The internet has loads of complaints about user selected default apps being reset to the OS defaults.

• Menu app icon > Preferences > General
• Under 'System Integration' section
• Select checkbox 'Always check to see if Thunderbird is the default mail client on startup'

What happens if you click on on the 'check now' button ?