How to disable "Set as Desktop Background" as an option in Firefox using Group Policy

Find the below simple steps.

1) To create file userChrome.css with below mentioned contents.

/*
* Edit this file and copy it as userChrome.css into your
* profile-directory/chrome/
*/


/*
* This file can be used to customize the look of Mozilla's user interface
* You should consider using !important on rules which you want to
* override default settings.
*/

/*
* Do not remove the @namespace line -- it's required for correct functioning
*/

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */
#context-setDesktopBackground {display: none !important;}
/* Tools > Clear Recent History - hide */
#sanitizeItem { display: none !important }

/* Tools > Options > Privacy - hide clear recent history options */
#clearDataBox { display: none !important }

/* Remove Sidebar History Searchbox */
#history-panel > hbox {display:none !important;}

/* Remove Organize bookmarks */
#bookmarksShowAll {display:none !important;}

/* Remove Items from History Menu */
menupopup > menuitem[label="Home"] {display:none !important;}
menupopup > menuitem[label="Show All History"] {display:none !important;}
menupopup > menuitem[label="Back"] {display:none !important;}
menupopup > menuitem[label="Forward"] {display:none !important;}
menupopup > menu[label="Recently Closed Tabs"] {display:none !important;}
menupopup > menu[label="Recently Closed Windows"] {display:none !important;}
menupopup > menuseparator[id="startHistorySeparator"] {display:none !important;}
menupopup > menuseparator[id="endHistorySeparator"] {display:none !important;}

/*
* Some possible accessibility enhancements:
*/

/*
* Make all the default font sizes 20 pt:
*
* * {
* font-size: 20pt !important
* }
*/
/*

* Give the Location (URL) Bar a fixed-width font
*
* #urlbar {
* font-family: monospace !important;
* }
*/
/*
* For more examples see http://www.mozilla.org/unix/customizing.html
*/

2) userChrome.css file should be saved in domain share drive.

3) Then Create batch file with below mentioned contents.

set folder=%APPDATA%\Mozilla\Firefox\Profiles

if exist "%APPDATA%\Mozilla\Firefox" for /D %%F in ("%APPDATA%\Mozilla\Firefox\Profiles\*") do Xcopy /E \\share path\* %%F

4) Deploy this batch files via Group Policy. A very common way of doing so is User Logon scripts.