Check out the Basic level tweaking here

You can tweak your menus using userChrome.css file. userChrome.css is a CSS file that can be used to change the way Mozilla Firefox interface look. But to edit it you need to locate it first.

For Windows XP users:

C:\Documents and Settings\\Application Data\Mozilla\Firefox\Profiles\

For Windows Vista and Seven Users:

C:\Users\\AppData\Roaming\Mozilla\Firefox\Profiles\

Or

1. Press WIN + R and type %Appdata%
2. Then navigate to Mozilla\Firefox\Profiles\lpg7v146.default\chrome
3. Here create a file named “userChrome.css”

The easiest way to hide a menu item is using the Label attribute.

For example, if you want to hide the “View Page Source” menu item from the right click context menu than use

menuitem[label="View Page Source"] {display: none !important; }
How to Hide Menu Bar completely?

#toolbar-menubar { display: none !important; }
Hide selected browser menus

#navigator-toolbox menu[label="File"],
#navigator-toolbox menu[label="Edit"],
#navigator-toolbox menu[label="View"],
#navigator-toolbox menu[label="Go"],
#navigator-toolbox menu[label="Bookmarks"],
#navigator-toolbox menu[label="Tools"],
#navigator-toolbox menu[label="Help"]

{ display: none !important; }
Remove Items From Right Click Context Menu



#context-back,
#context-forward,
#context-reload,
#context-stop,
#context-sep-stop
{ display: none !important; }
After applying this code



So what are you gonna hide in Firefox Menu or what do you want to hide?

Source: beingPC.com