You cannot safely go back to standard release, so you should create the beta and make a copy of current profile to use with it. So, Copy your full profile to a new location. Start the beta with profile manager and select the copied profile. You shouldn't need to reenter anything (although I'm not sure about any accounts using app passwords or OAUTH2).

Still not completely sure on how to proceed. I'm on Linux, the profiles are in the ".thunderbird" directory. Each profile has a subdirectory, and there is a profiles.ini that lists them. How exactly do I make a copy of my current profile and offer it up to thunderbird beta?

Okay, I admit that I was thinking of Windows, since that's where I live. But I do have PC with Linux and remember struggling to add userControl.css to it. My suggestion is similar to Windows. My suggestion, not being a guru with Linux, would be to: - copy the profile under .thunderbird to another place. I think command is cp -r (to include subdirectories) - after copying, rename it to something meaningful, e.g., BETAprofile - copy back to the .thunderbird folder. - At this point, I think you can start the beta with profile manager and that should allow to select the profile for the beta, which updates the profiles.ini for you.

Note: Those subdirectories you mentioned would be the actual account folders, e.g., Mail and/or Imapmail.

Let me know how it goes.