Starting from Fedora 33 or GNOME 3.38, X11 applications fail to start from inside a toolbox container. This is because Mutter stopped no longer listens on the abstract socket and only uses the file system socket to start the Xwayland server.
FEDORA-2020-12c4232ccb has been submitted as an update to Fedora 33. https://bodhi.fedoraproject.org/updates/FEDORA-2020-12c4232ccb
Setting as AcceptedFE as per voting: https://pagure.io/fedora-qa/blocker-review/issue/162
FEDORA-2020-12c4232ccb has been pushed to the Fedora 33 stable repository. If problem still persists, please make note of it in this bug report.