This is a plugin I found recently and really enjoy. It gives you the opportunity to redirect several webpages to alternatives. For example Youtube to FreeTube or Piped. If I search for youtube in Firefox, FreeTube automatically opens and shows me my feed. I like it, I use it, I can recommend it.
Addon files (.xpi files) are zip packages of the addons. They should contain the script files without obfuscation (I think this is an AMO policy), besides any resources and the addon manifest file.
The only thing that would be harder to inspect I think is webassembly files.