about:preferences shows
Your organization has disabled the ability to change some preferences.
How can I see what was disabled?
I compiled Mozilla Firefox 67.0 from source (via ebuild on Gentoo Linux) on my personal computer and I did not disable anything by intention.
Access about:config, sort by the "Status" column, and look for settings where status is 'locked'.
In particular, to determine the origin of those settings, look for general.config.filename which is the traditional Firefox autoconfig mechanism:
Recently Firefox gained a second, simpler mechanism – search for a file policies.json in your system; its contents will directly tell you what has been disabled:
Finally, some preferences might have been disabled by add-ons you have installed. There's probably no central method for listing those.
