A long time ago, Thunderbird could export all your Message Filters along with all your e-mails. It was great because you could copy all your information to another computer as quickly as a few seconds (minutes if you had many e-mails accounts and/or thousands of messages). Then, Mozilla came up with a migration scheme that would allow you to access your profile on another machine and import everything from the profile on your previous machine to a new install of Thunderbird on a new machine. In a very short time, you could have a complete clone of your Thunderbird profile on a new machine.
These things need to come back. Exporting e-mails and folders, then importing into a new install, then having to re-create dozens or even hundreds of Message Filters is unnecessarily tedious, given how flexible Thunderbird used to be.