Please always allow your users to alter the about:config settings! Like our useragent or any other information that we should be allowed to change! I just update to the new version of firefox and I tried to use the general.useragent.override value on the updated fox and it didn’t allow me to save it! Everytime I exited and reopened the value I set disappeared I even did it with a version of firefox I have NO add ons installed. ANd it still deleted the value I set.
Please never do this.
Thank you!
A loyal Comodo user for about 4-5 years. 
Hello,
Take a look at the following option in Firefox or IceDragon: general.useragent.enable_overrides.
Having it set to “true” should save the general.useragent.override option.
Note: Make sure you don’t have any user agent switching add-ons installed. The option is reset if the “User Agent switcher” add-on is installed, it might be the same for other add-on as well.
Regards.