Hi,
You can easily view the source code of ANY extension on Firefox. They are just simple ZIP files with XPI extension. First, rename the extension from ".xpi" to ".zip" and then extract.

You can read more from here (or just google something like "check firefox extension source code"):
https://superuser.com/questions/771825/how-to-examine-source-code-of-firefox-extension-before-installing-it)

Anything in Open Source doesn't mean that it is safe. Because the one in the repository may NOT be the SAME with the one you INSTALL. This is a simple TRAP that many people will get in.