Tools to monitor App files write activity under OS X
Instruments is a tool that comes with Xcode.  This tool was designed to help developers get a lot of detail about what was happening on a computer when their software is run.  One of the things it can do is watch all file I/O.  If you run the program and add the File I/O instrument, you can attach it to safari and see what happens.  You can also filter and sort the output.

More information can be found on the Xcode page from Apple.