File System access from Firefox Extension

Is it possbile to create a fir开发者_JAVA百科efox extendsion to write to files in a windows file system ?

Yes, see this page for examples.

Edit

XPCOM and so nsIFile are now legacy technologies :

Use of OS.File is preferred over the examples in this article. Only use these legacy interfaces if OS.File is not available to you.

You can find the new way to go here