Maybe the Thunderbird addon GMail conversation view produces the desired result.
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That's a really good start thanks, but I'm really looking for something which would group all the messages with my ex's email adress, no matter the topic.
Because with this extansion, it's still related to the topic of the email.
In thunderbird there is an option to always add your own email to the bcc list. This will put all your replies directly into your inbox and thread them correctly in the conversation.
I’m gonna check this, thanks!
If you open your All Mail folder instead of your Inbox it should group all the emails (sent and received) together. However, when I try, it duplicates many emails, so it is confusing.
I don't have an “All Mail” folder in thunderbird. Maybe this is something specific to your email provider?
Every email provider I have used has had an "All mail" folder. Perhaps it is called Archive by some providers.
However, you could create such a "folder" yourself. I'd suggest a tag actually. Set up a filter in thunderbird or via your email provider to tag all received emails and all sent emails with this tag. You can then open the tag to see all emails.