This browser uses the same GeckoView engine as the recently updated Firefox Focus (for Android), which runs much faster than the Gecko engine that's currently being used in Firefox for Android.
As an Android engineer, what would I use this for in my app?
Is it something I would embed to use in lieu of Chrome Custom Tabs if I wanted an in-app browser experience (similar to CCT) but that I could customize more or if I wanted more control over the "web browser engine" for some reason?
Mozilla's Android components repo describes the project as "A collection of Android libraries to build browsers or browser-like applications." Since this includes an entire browser engine and browser features such as autocomplete and history/bookmark syncing, I don't think this is meant to be a substitute for Chrome Custom Tabs. If you intend to develop a complete browser (e.g. Brave) or an app that uses multiple browser features (e.g. a shopping app with a built-in browser), then Mozilla's Android components might be right for you.
The Mozilla Reference Browser appears to be targeted to end users as a way to showcase performance improvements in an upcoming version of Firefox. Right now, Mozilla isn't able to include GeckoView into Firefox Nightly because it doesn't support all of the existing browser features.
Is there a similar component for the Firefox JS engine? I'd be interested to know if it could be used standalone and how big it would be. (Mozilla also has the Rhino JS engine that works on Android, but this is something different)
Originally, there was XULRunner, which was used in applications including ChatZilla, Google AdWords Editor, Komodo Edit, Pencil, Songbird, and Zotero. XULRunner has been discontinued.
Mozilla briefly developed Positron, which was an Electron-compatible app shell that used Gecko instead of Chromium/Node.js/V8. Positron has also been discontinued.
The developer who was working on Positron went on to develop qbrt, which is a CLI interface to a Gecko app runtime. The project is unstable, and the last commit date is Jun 30, 2018.
Another Mozilla developer has a project called servoshell, which is an embeddable version of Servo. It's described as work-in-progress, and the last commit date is Jan 7, 2018.
https://support.mozilla.org/en-US/kb/geckoview-firefox-focus