is there a common way in Mozilla and Firefox to copy additional files in install time?

In Firefox I understand that the additional files that are not 
Preferences (e.g. some example files) can go to Defaults folder when 
installing a package.

So I started using Defaults instead of Chrome folder and run into problems.

Firefox install.rdf understands the Defaults structure from the .xpi 
file (at least in documentation), but in Mozilla install.js I cannot 
find how I should refer to that folder with getFolder(). DefRt does not 
seem to work.

Please help!

Marja