omni.ja is a file in the Firefox program directory. As a Windows person, I don't know where that is on a Mac.

This file is replaced every time Firefox updates, so it's not very practical to edit it. What do you need to change?

The two omni.ja archive files are signed (META-INF folder) and are packed specially to allow loading all files as quickly as possible, you can't edit these file as that invalidates the signing.

What changes would you like to make ?