Its is the Mozilla way of saying in a humorous way that no bugs were found. When you search in Mozilla bug database (BugZilla), and if, no records were found, instead of returning "0 bugs found", it returns "Zarro Boogs". It is a facetious metastatement about the state of software in development. Note that the response "Zarro Boogs" itself is a misspelling of "Zero Bugs", which indicates that, though no bugs were found, the software is likely to contain bugs that have not been identified yet.