I don't expect software to (significantly?) change during runtime, outside of what was packaged, signed, distributed and installed as part of apt/yum/pacman/etc.
I understand (not that I like or agree with) that some apps are just embedded web browsers, and load everything externally, and that Firefox extensions are in the end just some JS/CSS/HTML loaded outside of system's package manager. However, extensions have limited API they can interact with, and you need to allow permissions for each extension. Having Mozilla owned extension, that can modify core functionality, seems a bit scary.