There are various ways to manipulate single entries, but since you want to restrict everything as much as any possible: go straight to the source of all :cool:
...browser/defaults/settings/
in
menus.cfg most menu entries are created, except very few by macros and plugins. The menus are structured modular, with parent menus and submenus inside them (groups which either pop up with an arrow, or are completely visible inside the parent menu) and single menu lines.
There are also "context" menus which pop up on right-mouseclick, they are also making use of some submenus created above already.
The whole menu tree must be build bottom-up, all submenus must already be created above before creating their parent menus further down.
=> menus and single lines can be disabled by adding a # before the line. Or test how it look after removing the whole cfg-file...
And in the same folder take a look at
accels.cfg, same system for keyboard shortcuts.
Some more macro-created menus and shortcuts are inside the kmm-files in the macros folder. Look for "setmenu" and "setaccel" commands in them.