This is a Sizzle bug specifically, but I'm not sure if this is the correct issue tracker.

Can be reproduced with a simple test case:

jQuery.find.matchesSelector($('input')[0], ':input')
// returns false

On old FF, invalid selectors passed to mozMatchesSelector would always return false instead of correctly throwing an exception. There used to be a feature detect in Sizzle to test for this.

// This should fail with an exception
// Gecko does not error, returns false instead
try {
  matches.call( div, "[test!='']:sizzle" );
  rbuggyMatches.push( Expr.match.PSEUDO );
} catch ( e ) {}