[frogjerk]

If your account connects using POP, then deleting server copies when you download messages is the default behaviour, unless you specifically change it by setting the checkbox "Leave a copy on server".This can be found under Account Settings. If your account uses IMAP then it will by its nature leave copies on the server. If you want to clear the server, move the messages you want to keep to the Local Folders account in Thunderbird. Doing this will erase the server copies. For messages you don't want to keep, just delete them in Thunderbird. IMAP synchronization will delete the server copies too. Read this answer in context 👍 2

https://support.mozilla.org/en-US/questions/1119213

[LesbianThespianGymnasticMidget]

IMAP

Check into if that is what you want.

https://www.youtube.com/watch?v=SBaARws0hy4

[daniel1212]

IMAP is said to be better, but you seem to use POP. Go to your gmail account online, and hit the gear icon on the top right, then hit the Forwarding and Pop/IMAP tab, and select your preferences.

In T-bird, go to Tools>Account settings and hit Server settings, then choose what you want.

[Revel]

In Thuderbird click on your account name in the left panel. From the screen that pops up Choose “View settings” under account. (I believe you can also get to this by right clicking on the account name and choosing the option.

Now in the menu that pops up then select “Server Settings” under the account name.

Now uncheck the box that says “Leave Messages on server”.

This is for Pop accounts only.