Does firefox sync() the data? If not, these continuous overwrites of the same file may not even hit the disk at all, as it could all end up being cached.

Even if some data is being written, it could still be orders of magnitude lower than the writes executed by the program.

There are legitimate pros/cons of using sync() or not. Missing it out could mean that the file data is lost if your computer crashes. But if firefox crashes by itself, the data will be safe.