A question about customization and source code.

 I'm curious if there is an avenue to customize FF, first for desktop and maybe the mobile,  and possibly even enterprise if accepted, via source. If it is possible, where exactly can I find the code? This is a preliminary question and I do plan to check git. The reason I'm starting here, one may ask of me. The answer is simple enough, like any other enthusiast dev, time is a resource best spent doing something you enjoy. So, where better to ask than the experts and community who may have a link on speed dial. 

To begin with, as there isn't an option in the drop down, I thought settings would be the best place to begin. Hi, I'm curious if there is an avenue to customize FF, first for desktop and maybe the mobile, and possibly even enterprise if accepted, via source. If it is possible, where exactly can I find the code? This is a preliminary question and I do plan to check git. The reason I'm starting here, one may ask of me. The answer is simple enough, like any other enthusiast dev, time is a resource best spent doing something you enjoy. So, where better to ask than the experts and community who may have a link on speed dial. Thank you all for any help!