How to set global/system Firefox prefs for new users?

[John Hupp]

For Firefox in newly added user profiles, I have a few modest 
preconfiguration goals: 1) Customize the toolbar to add Print, History, 
and Tab Groups.  2) Set new tabs to display a blank page.

I would think that #1 might be accomplished either by setting
     pref("browser.uiCustomization.state", "{long string copied from my 
user profile pref}");

or else by copying my initial user's xulstore.json to 
/usr/lib/firefox/browser/defaults/profile or 
/usr/lib/firefox/defaults/profile.  (Or by doing setting the pref AND 
copying xulstore.json.)

But to zero in on setting the prefs: I failed first with what I took to 
be the standard Firefox methods.

Then I found 
https://help.ubuntu.com/community/UbuntuLTSP/FirefoxOptimize, and though 
it was written for *buntu 12.04, I'm assuming that it's probably still 
valid for 14.04.  (I'm running Lubuntu 14.04.1 32-bit.)  And it 
indicates that global/system prefs should be placed in 
/etc/xul-ext/ubufox.js.  But my prefs seem to be either ignored or 
subsequently overridden/overwritten.

(And yes, xul-ext-ubufox is indeed installed.)

How do you successfully do these things in 14.04?