file: URLs (i.e., files on the filesystem. However, to use a file packaged inside the extension, see below)
privileged about: URLs (for example, about:config, about:addons, about:debugging).
Non-privileged URLs (e.g., about:blank) are allowed.
The New Tab page (about:newtab) can be opened if no value for URL is provided.
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/create
Closes #103