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Selecting previously unselected package squashfs-tools. (Reading database ... 387975 files and directories currently installed.) Preparing to unpack .../squashfs-tools_1%3a4.3-12_amd64.deb ... Unpacking squashfs-tools (1:4.3-12) ... Preparing to unpack .../snapd_2.37.4-1+b1_amd64.deb ... Unpacking snapd (2.37.4-1+b1) ... Setting up squashfs-tools (1:4.3-12) ... Setting up snapd (2.37.4-1+b1) ... Processing triggers for mime-support (3.62) ... Processing triggers for man-db (2.8.5-2) ... Processing triggers for desktop-file-utils (0.23-4) ... epp@upstairs:~$ sudo snap install core snap "core" is already installed, see 'snap help refresh' epp@upstairs:~$ sudo snap install thunderbird error: cannot perform the following tasks: - Mount snap "thunderbird" (124) (snap "thunderbird" assumes unsupported features: snapd2.43 (try to update snapd and refresh the core snap)) epp@upstairs:~$ sudo snap install thunderbird error: cannot perform the following tasks: - Mount snap "thunderbird" (124) (snap "thunderbird" assumes unsupported features: snapd2.43 (try to update snapd and refresh the core snap)) epp@upstairs:~$ sudo apt update snapd E: The update command takes no arguments epp@upstairs:~$ sdudo snap remove thunderbird bash: sdudo: command not found epp@upstairs:~$ sudo snap remove thunderbird snap "thunderbird" is not installed epp@upstairs:~$ sudo snap install thunderbird error: cannot perform the following tasks: - Mount snap "thunderbird" (124) (snap "thunderbird" assumes unsupported features: snapd2.43 (try to update snapd and refresh the core snap)) epp@upstairs:~$ sudo apt refresh snapd E: Invalid operation refresh epp@upstairs:~$ sudo apt refresh core E: Invalid operation refresh epp@upstairs:~$ sudo snap refresh core snap "core" has no updates available epp@upstairs:~$ sudo snap install thunderbird error: cannot perform the following tasks: - Mount snap "thunderbird" (124) (snap "thunderbird" assumes unsupported features: snapd2.43 (try to update snapd and refresh the core snap)) epp@upstairs:~$ sudo apt update snapd E: The update command takes no arguments epp@upstairs:~$ sudo apt upgrade Reading package lists... Done Building dependency tree Reading state information... Done Calculating upgrade... Done The following packages were automatically installed and are no longer required:   libbotan-2-9 libtspi1 Use 'sudo apt autoremove' to remove them. 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded. epp@upstairs:~$ sudo snap remove thunderbird snap "thunderbird" is not installed

I was able to get Thunderbird installed via the Ubuntuzilla repository at SourceForge. It shows it's on the Release update channel, so I trust it will update automatically from now on.