We will stick with this version of firefox until no longer supported by Mozilla.
Eventually all versions of Simple Browser will be firefox based (Simple Browser C# will still be updated and supported)
An explanation of the Firefox Source Code Directory Structure and links to project pages with documentation can be found on the relevent page on firefox-source-docs. (Will be updated with more elaborate info soon)
For information on how to build Simple Browser from the source code see the Wiki.
If you have a question about developing Firefox, and can't find the solution on this repo, visit the firefox-source-docs
If you still need help, you can also try asking your question on Mozilla Matrix at chat.mozilla.org if needed.
Note
Keep in mind that nightly builds, which are used by DMC developers for development, and by Firefox developers for testing, may be buggy.