As stated in the Firebug discussion group, Firebug’s Console panel currently (Firebug 2.x) cannot filter single messages out. You can only filter by messages matching a specific text using the search field.
There are enhancement requests like issue #4507 reported to add such a filter, though it is unlikely that this will get implemented in Firebug, especially not in version 2.x, directly.
Firebug 3 will integrate into the built-in DevTools, which allow you to filter those warnings by unchecking Warnings within the Security menu inside the Console panel:
Besides that I created bug 1170476 asking to reduce the messages to a minimum.
Update:
Firebug development is discontinued. Instead, there is a Firebug theme available within the Firefox DevTools starting from Firefox 48. Bug 1170476 is fixed since Firefox 42, so you will only see one message logged with a counter showing how often the message occurred.