Checked on my work computer and you're right! Something must be going on with my userChrome.css back at home that's inadvertently hiding it from me, since I hide useless context menu options like "Set as Desktop Background...", "Email Image", etc. Something I'm hiding must be hiding "Add as a Keyword for this Search..." on accident.
I wish :contains() had stayed in CSS Selectors Level 3, or that you could use XPath in stylesheets.
Hmm… #contentAreaContextMenu actually looks pretty sane. They all have IDs, which I really should have expected anyway. I’m going to add some rules to my own userChrome.css. Thanks for the idea!