Your question confuses me. I t seems you want to keep the messages in Gmail, but not on Thunderbird? If that is the desire, then I suggest you define a new label in Gmail and move all the messages received prior to 2021 there and then specify via Gmail to now allow the label to be shared with any email apps.

Thanks .... Yes I understand that I put messages that I want to keep in a different folder.

I would like to delete the thousands of old messages with one click .... But takes too long in Gmail as I have to do it page by page ... I was thinking that Thunderbird might have a way to do that.

Thanks again.

Thanks .... Yes I understand that I put messages that I want to keep in a different folder.

I would like to delete the thousands of old messages with one click .... But takes too long in Gmail as I have to do it page by page ... I was thinking that Thunderbird might have a way to do that.

Thanks again.

Have you tried the windows shortcuts that have Worked since Windows 3, back in 1985. It will not be one click, but Ctrl+A then pressing the delete will delete everything in the folder. Almost as fast, unless you don't have a keyboard.

Ctrl + A to select all Ctrl + Shift + click to select everything between where you are already showing a selected item and where you click Ctrl+Click to only affect the clicked item toggling it's selection status with each click without modifying other selected items in the list.