when I delete messages directly from gmail, I see that they are automatically deleted from Thunderbird as well, how can I prevent this from happening again?

You can't. This is how IMAP works.

I use gmail as an email client and I want to use Thunderbird to save all my emails there.

What are you trying to achieve in the first place?

Hello @christ1, thanks for reply. So what is alternative then for IMAP? I had a google account, I lost it, now I want to have all my email saved on my thunderbird, and when I delete something from gmail account, to still have those email on my thunderbird, independetly. I try to achive, to prevent data lost, and to save my emails, in thunderbird, then to delete from gmail website. Because I lost an email address in the past, I don't want to repeat the unpleasant experience with gmail again. So I want all emails to be saved independently of gmail in Thunderbird, and then to delete them from gmail, but to remain only in Thunderbird and not to be deleted from Thunderbird. When I delete something from Gmail, the emails from the Thunderbird account should not be affected, because in the past I lost all the emails.

You'd have to use POP instead of IMAP. Then you'd also have to manually replicate any folder hierarchy locally you may have on the IMAP server.

Alternatively you can move messages manually off of the server to your Local Folders account, and use it for archiving purposes. Same thing as for POP, you'd have to manually create the folder hierarchy you want.

Or take a look at an email archiving software like MailStore. https://www.mailstore.com/en/products/mailstore-home/