Just to check, you definitely mean Firefox**OS** the mobile operating system, not Firefox the web browser?
What would you like to do with FirefoxOS/B2GOS? Do you already have a device which is running FirefoxOS? Or do you have a device you’d like to install FirefoxOS on?
Which guides have you been reading? The archived documents in MDN https://developer.mozilla.org/en-US/docs/Archive/B2G_OS are still relevant, though I agree they’re maybe not as accessible as they could be. The Mozilla Wiki is probably even less accessible/coherent, unfortunately. This is all a symptom of the recent history of the project, which is that Mozilla started to “transition” the project to community ownership in late 2015 then decided to completely abandon it in September 2016. The Mozilla codebase was gutted of B2G-related code to make it almost impossible to continue the project in it’s state at that time. Naturally, the documentation was left as a snapshot, partially through the transition to community ownership. (although I think some will say the transition was officially completed).
There are still many people interested in progressing the project however, and some similar projects in progress, e.g. Servonk.
In terms of building and installing B2GOS (the latest versions) these instructions are still correct, though I can’t verify everything: https://developer.mozilla.org/en-US/docs/Archive/B2G_OS/Building_and_installing_B2G_OS
Let us know if this is what you meant or something else.