Fire Previewfox is Mozilla’s new Android web browser in the first half of 2019. The browser’s rendering engine, GeckoView, was developed by Mozilla itself, and by separating the GeckoView engine from the Firefox application, the development team created an updated, faster, and easier way to develop Android applications.
Like the desktop version of Firefox, Firefox Preview offers different versions such as Beta and Nightly. In general, the core functionality in different versions does not vary much. Recently, however, developers discovered that Firefox Preview’s Stable and Beta versions: config is no longer supported.
Config is an internal page long supported by Firefox that provides an interface for advanced configuration changes directly from a web browser. So far, both desktop and mobile versions of Firefox have supported it as normal.
However, when a user tries to load an about:config in Firefox Preview’s Stable or Beta version, they receive a “request cannot be completed” message. The page says “Additional information about this issue or error is not currently available” and there is a “Try Again” button to reload the page.
It is not clear whether the change was intentional or whether Mozilla will restore the page before it is finally released. In summary, current Stable or Beta users cannot use about:config to change the configuration of Firefox Preview.
The reason for this appears to be found in the response seeking from the user’s previously proposed Issue:
This is the expected behavior of the GeckoView engine. FireFox does not control access to it. Users with this requirement can access the item:config through the nightly version.