We ve all been in endless email loops, with every correspondant quoting all the previous emails. This leads to very painful emails to read, and thunderbird does not collapse in-mail replies. Popular clients like gmail, or Samsung mail already implement this, which makes reading emails easier. It is particularly useful when our own sent emails are included in the conversation (but that feature was already suggested multiple times).
When this gets implemented, make the current email content only available to add-ons.
I have a 95%-working add-on that detects replies, but it still has some false-positives.