Cannot start GUI applications “Error: cannot open display: :0”

I am getting this error when trying to launch Thunderbird (or any other program, such as evince):

❯ thunderbird
Unable to init server: Unable to create socket: Permission denied
Error: cannot open display: :0

❯ thunderbird -v
Thunderbird 68.10.0

Any ideas?

❯ cat /etc/linuxmint/info
RELEASE=20
CODENAME=ulyana
EDITION="Cinnamon"
DESCRIPTION="Linux Mint 20 Ulyana"
DESKTOP=Gnome
TOOLKIT=GTK
NEW_FEATURES_URL=https://www.linuxmint.com/rel_ulyana_cinnamon_whatsnew.php
RELEASE_NOTES_URL=https://www.linuxmint.com/rel_ulyana_cinnamon.php
USER_GUIDE_URL=https://www.linuxmint.com/documentation.php
GRUB_TITLE=Linux Mint 20 Cinnamon

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Author: Philippe Massicotte