Mozilla Firefox Preview is in the first half of 2019 to launch a new Android Web browser. The rendering engine GeckoView browser developed by the Mozilla themselves, by GeckoView engine with Firefox application phase separation, the development team to create a newer, faster, and easier to maintain way to develop Android applications.
Like the desktop version of Firefox, Firefox Preview also provides different versions of the Beta and Nightly and so on. Typically, different versions of the core functions will not be much difference. But recently, developers have found Firefox Preview of the Stable and Beta version, about: config is no longer supported.
About: config is an internal page has long been supported by Firefox, which provides an interface for advanced configuration changes directly in the Web browser. So far, the desktop and mobile versions of Firefox support it are still normal.
But when the user attempts to load about in Stable or Beta version of Firefox Preview: The config, will receive a message "Unable to complete the request". Page display "on this issue or error Additional information currently unavailable", and a "retry" button to reload the page.
It is not clear whether this change is intentional, it is unclear whether Mozilla will return to the page before the final release. In short, the current Stable or Beta users can not use about: config to change the Firefox Preview configuration.
From users have previously proposed Issue responses obtained seem to find the reason:
This is expected behavior GeckoView engine. Firefox Preview does not control access to it. This requires users can access through the nightly version about: config.
Source: Ghacks