i am not sure if its still a thing but there was an app that automated this method for spying on your contacts. it even became quiet popular in the different app-stores.

But all of that is very limited data. Imagine what Meta itself can do with all the collected metadata. I wonder where their name came from

for TB -> TB-flatpack simply pasting from ~/.thunderbird/ to ~/.var/app/org.mozilla.Thunderbird/.thunderbird/ did work for me. I might try betterbird sometime.

They are very open about that