You may want to iterate rapidly with customizing the environment in Firefox Reality. In order to do so, you will need one developer tool (adb) and to change a few settings.

Please follow the instructions here for enabling Developer Mode on the Oculus Go.

As long as Enable Environment Override is enabled, Firefox Reality will look in the /storage/emulated/0/Android/data/org.mozilla.vrbrowser/files/skybox directory for .png, .jpg, or .ktx files.

Environments should be produced in a cubemap format. There is an example in-tree.

You will then push that directory to the appropriate place on the device using adb. An example of pushing the space skybox from the above directory is below:

[larsberg@Valkyrie cubemap]$ adb push space/ /storage/emulated/0/Android/data/org.mozilla.vrbrowser/files/skybox
space/: 6 files pushed. 20.1 MB/s (2796814 bytes in 0.132s)
[larsberg@Valkyrie cubemap]$ 

Or for quick testing:

FXR_SKYBOX_DIRECTORY=~/Downloads/skybox/ && \
        adb shell rm -rf /storage/emulated/0/Android/data/org.mozilla.vrbrowser/files/skybox && \                       
        adb push $FXR_SKYBOX_DIRECTORY /storage/emulated/0/Android/data/org.mozilla.vrbrowser/files/skybox && \
        adb shell ls /storage/emulated/0/Android/data/org.mozilla.vrbrowser/files/skybox

Launch Firefox Reality. In the Settings panel, select Environments, and turn on Enable Environment Override. The new custom environment should now be displayed.