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I’ll note that a number of groups and forums send mailing list like emails (google groups, django dev being a big one) and that notifications can be threaded from places like Github with the right client.
Thunderbird has good threading.
Roundcube webmail is also capable here. Though when I have had it working it didn’t include sent messages… which is not great in my mind.
The deb version is a pointer to the snap in their repos. Nothings being replaced, it no longer exists. The deb version of Firefox in Ubuntu repos is a wrapper that installs snap and has no binaries in it. Has been for 3 years or so.