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Next Unread Group not be installed because it is not compatible with TB 115.8.1 (latest version). Please update this add on.

Ta opinia jest o wcześniejszej wersji dodatku (1.2). 

It should currently work with Thunderbird up to 125.*

You can try to update or install it from within Thunderbird:
Go to the Add-ons Manager and search for Next Unread Group.