Preference editing has not yet been re-implemented as part of the WebExtension re-write.
If you're feeling brave in the meantime, you can edit the preferences in their JSON file: click the “hamburger” menu button, help “(?)”, “Troubleshooting Information”, and then under “Application Basics” select “Show Folder” or “Open Directory”. This is your Firefox profile. Go into the browser-extension-data/{139a120b-c2ea-41d2-bf70-542d9f063dfd} directory and the preferences are there.
Preference editing has not yet been re-implemented as part of the WebExtension re-write.
If you're feeling brave in the meantime, you can edit the preferences in their JSON file: click the “hamburger” menu button, help “(?)”, “Troubleshooting Information”, and then under “Application Basics” select “Show Folder” or “Open Directory”. This is your Firefox profile. Go into the browser-extension-data/{139a120b-c2ea-41d2-bf70-542d9f063dfd} directory and the preferences are there.
Make a backup before editing!