Sort of? But the amount of effort required is much higher. Source patches require having a full dev environment, it takes much longer to build the entire browse

Sort of? But the amount of effort required is much higher. Source patches require having a full dev environment, it takes much longer to build the entire browser and you also have to take on responsibility for rebasing onto upstream, dealing with security releases for the browser itself _and_ every one of the patches, and running an update infrastructure. Extensions from other people (which is 90%+ of them even for extension developers) just need to be loaded, and your own extensions only need a text editor and something that can make zip files. Maintaining Firefox build and update environments is a multiple-employee full-time job at Mozilla, and that's before you've even started modifying anything. It's just too much to expect an end user to do this (and even extension developers are end users most of the time).