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How to disable IPv6 on a specific interface in Linux?
How do I turn off IPv6 in Thunderbird?
Procedures
In Thunderbird, click on the Settings option in the program's main drop-down menu. ...
In the search Find in Settings box, type network.dns.dis -- press Enter.
The first option should be network.dns.disableIPv6 -- select this and toggle to change it from false to true.
Close and reopen Thunderbird.