I’m a heavy Thunderbird user and to be honest, I don’t understand what you’re saying at all? I have multiple private mail accounts and a work mail account and I use all of them on multiple machines with Thunderbird but also with different clients (e.g. FairEmail on Android) as well as webmail (at least for my work mail I use it sometimes) and I never experienced any problems. What exactly do you mean? I mean, I do have an export of my thunderbird profiles (maybe not up to date, though, tbh), but more so out of comfort than necessity. Without this export, and in the unlikely case of a system failure, I would have to go through the process of adding my mail accounts (server, password, username) by hand and that’s basically it