This cleanup has been done (by some clever robots here) to eliminate the old style of octal literals (0123) which are not allowed in strict closure compilation mode, basically to improve the range of compilation settings this will compile with.

As for the nsIFileInputStream#init method and this parameter in general, my understanding it that's the 'mode' parameter here https://developer.mozilla.org/en-US/docs/PR_Open#Parameters and octal 666 means read and write permission to the owner, group and the others.