Hi
That setting is not anywhere in the regular policy plist settings example file that Mozilla publishes:
policy-templates/mac/org.mozilla.firefox.plist at master · mozilla/policy-templates · GitHub

However, they have some additional references/examples to supported payloads which includes network prefix payloads. 
GitHub - mozilla/policy-templates: Policy Templates for Firefox 

@hari_premasiri were you able to get this setting configured? 

@karengarner Did you managed to ever get this working?  Looking for same solution...

Hi, Anyone find the way to disable Quic protocol in Firefox?

@karengarner  & @shrisivakumaran 

Sorry for the late response, 

Unfortunately, I have looked through Mozilla's documentation on plist and have not been able to find anything to restrict this setting available yet.
https://github.com/mozilla/policy-templates/blob/master/mac/org.mozilla.firefox.plist

This might have to be a feature request on Mozilla's side to allow control over this setting.

Thanks, I got the solution from Mozilla Support forum page. now able to control it

Would you be willing to share how that you are controlling this? 

@timbyler1890 using below plist to disable quic protocol in Firefox.

<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"> <plist version="1.0"> <dict> <key>EnterprisePoliciesEnabled</key> <true/> <key>Preferences</key> <dict> <key>network.http.http3.enable_0rtt</key> <dict> <key>Value</key> <false/> <key>Status</key> <string>locked</string> </dict> <key>network.http.http3.enable</key> <dict> <key>Value</key> <false/> <key>Status</key> <string>locked</string> </dict> </dict> </dict> </plist>

Thanks a lot for that info.

That is what I was going to try but that should save me a lot of time.