I also have this problem. As far as I can tell, it happens when other add-ons also add entries to the context menu.
For example, I can reproduce it in a new profile by doing this:
1. Install Adblock Plus and the Image Zoom add-on: https://addons.mozilla.org/en-us/firefo ... mage-zoom/
2. Go to http://www.ebay.com/sch/
3. Right-click the eBay logo and select Adblock Plus: Block image...
4. Nothing happens.
5. Disable Image Zoom and try again. It now works.
I can also reproduce the problem using NoScript instead of Image Zoom.