I updated my Macbook Pro (2017) but now my Thunderbird Mail-Client does not work - it is in way frozen
What can I do making my Thunderbird work under OS Big Sur?
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Were you able to look through Mozilla Thunderbird support site, & see what
may be newest for Big Sur? There's several updates to Big Sur since its release.
There is a 'security protocol' that may interfere with installing this software
and you may have to disable it in order to proceed; then turn it back on.
[While Thunderbird has remained an ESR product; presently at v.78. Updates
usually are an annual event, based on a tested-stable release.]
A changed feature may also affect use of this email client in Big Sur:
Mailfence Encrypted Email Suite in Thunderbird
The security in Big Sur is likely to interfere with Thunderbird. There may not
be 'fixed update/upgrade for Thunderbird to run under Big Sur. This client
had been an ESR development; subject at times to being dropped by Mozilla.
[It's said Thunderbird release ESR, roughly follows 'Firefox ESR' yearly release.]
https://www.thunderbird.net/en-US/contact/
• Thunderbird Help (searchable) - support.mozilla
https://support.mozilla.org/en-US/products/thunderbird
There are further questions about Thunderbird in Big Sur, after Catalina.
Suggest you contact Mozilla support about these issues, for answers.