If your context menu in Firefox is long enough to trouble you and you want to slim and trim it, here is a nice add-on for you : Ffchrome.
FfChrome is an extension that slims and trims your context menus on the fly for simplicity, ease of use, easy access and eliminates the clutter. Menus show only the essential items by default. You can hover the mouse on “Show All” and menus expand on the fly.
You can decide the items to be displayed and also set the time delay before menu expands automatically.
A nice firefox add-on which will make your Firefox experience better!