Composed messages can be stored in the Outbox for later sending.
Standard Thunderbird (for a long time and also up to and including V115) has facilitated later sending via Send Unsent accessible from the File Menu or right click on the Outbox.
Prior to V115 the Get/Send Button extension provided a Mail Toolbar button that would, with a single click, send unsent messages from the Outbox (as well as Get messages for the currently selected account). The Get/Send Button extension also provided a drop down list with more granular options, including Send All Unsent.
Get/Send Button is no longer supported with V115 (not surprising as much of its functionality has been incorporated into V115 - but not Send Unsent).
Thunderbird enhancement now requested to fill the gap left by demise of Get/Send Button
It would be much smoother and more consistent if Thunderbird could now incorporate a Send Unsent button that could be available for placing on the Mail Toolbar. I assume that this should be fairly straightforward as such a button would, surely, merely provide another path into the Send Unsent logic that is already provided.
Hope that's all clear.