The sender could rename the files before attaching them. I don't see how Gmail has anything to do with this.

No matter, you could change Thunderbird's settings so that it always asks where to save the attachments. That option is in Settings > Files & Attachments. When you select Save all, it will ask for a location to save those files. You will then be able to choose a different location for each message that you want to save the attachments from. For example; Message 1 with 50 attachments saved in Folder 1, Message 2's attachments in Folder 2 etc. I am assuming that the attachments in any given message have unique names.

No, they do not have unique names. All 50 attachments in each mail have the same file name, that's the problem which Thunderbird can't handle by itself (like automatically adding numbers to the file names).

Interesting, Thunderbird handles the same problem very well when I save multiple emails to files ("save as…") and it happens that 2 or more mails have the exact same subject and time (which the file name is made of), then the second file will have "-2" at the end of the filename before the dot, the third "-3" and so on.

The reason why those attachments that I received have the same name and why the sender can't just change them is secondary. The attachments are emails forwarded as attachments, which all have the same subject thus the same file name.

The inbuilt Save as facility is not available for saving multiple attachments, unlike saving multiple messages to files. The Save all facility for attachments lacks the auto-increment-numbering that the Save as facility offers. You could share that idea here https://connect.mozilla.org. Alternatively, the ImportExportTools NG add-on seems to support extended naming options for both messages and attachments. Let me know what version of Thunderbird you're running.

Hello Stans, thanks for the idea. I am currently using Thunderbird 102.13.0 (I decided to wait for the brand new Thunderbird to be available in Ubuntu repositories).

EDIT: I tried to do something with ImportExportTools NG but I can not find a way to save single emails and attachments with it. Export the whole folder and then find the attachments?

I'm not sure if the Thunderbird-102-compatible version of ImportExportTools NG has this facility, so you'll have to check as follows:

1. Open Thunderbird's menu and go to Add-ons and Themes. Click the wrench (spanner) icon next to ImportExportTools NG to open its options dialog.

2. Switch to the Filenames tab.

3. Under the Attachment Folder Names section, right next to the Attachment Folders field, do you see a white question mark circled in orange?