about:config is not available in Firefox Preview
Firefox Preview is a new Android web browser launched by Mozilla in the first half of 2019. Like the desktop version of Firefox, the Firefox Preview is also available in different versions such as Beta and Nightly. In general, there will not be much difference in core functionality between different versions. However, recently, developers found that about:config is no longer supported in the Stable and Beta versions of Firefox Preview.
about:config is an internal page that has been supported by Firefox for a long time. It provides an interface to make advanced configuration changes directly in your web browser. So far, desktop and mobile versions of Firefox still support it normally.
But when users try to load about: config in the Stable or Beta version of Firefox Preview, they receive a “Cannot complete request” message. The page says “additional information about this problem or error is currently unavailable” and a “try again” button to reload the page.
It is unclear whether this change was intentional or whether Mozilla will restore the page before its final release. In short, current Stable or Beta users cannot use about: config to change the configuration of Firefox Preview.
Source: ghacks
