alt Hacker NewsMozilla tells you to use it so it so that seems supported enough to me (example: https://support.mozilla.org/en-US/kb/how-stop-firefox-making...)
That said, they're admittedly terrible about keeping their documentation updated, letting users know about added/depreciated settings, and they've even been known to go in and modify settings after you've explicitly changed them from defaults, so the PSA isn't entirely unjustified.
Ugh. Because they also say:
"Two other forms of advanced configuration allow even further customization: about:config preference modifications and userChrome.css or userContent.css custom style rules. However, Mozilla highly recommends that only the developers consider these customizations, as they could cause unexpected behavior or even break Firefox. Firefox is a work in progress and, to allow for continuous innovation, Mozilla cannot guarantee that future updates won’t impact these customizations."
https://support.mozilla.org/en-US/kb/firefox-advanced-custom...