In Firefox, you see empty entries in the context menu that is displayed when you right-click somewhere on a webpage.
This is a bug typically caused by the addons that you are using with Firefox. It shows up after you upgraded the version of Firefox OR added a new addon OR upgraded an existing addon. So, the solution is straightforward: disable all addons and then enable them one-by-one to figure out the offending addon. Disable the culprit addon until the developer of the addon releases a new version that fixes this bug.
Tried with: Firefox 6