Proposed function:
This function in Thunderbird would allow showing all emails from the sender of which e-mail is currently selected.
Use case:
Activation:
It could be activated either through keyboard shortcut (as in the Vivaldi email module), which works as quick message filter directly in the message list; or it is dedicated view over the Today panel, showing list of messages, which is case for em-client.
Bottom line:
This is a function which always makes me return to the one of the mentioned products.