Having a simple text-based config with a repo, like [AddOns] ensure-ffpkg AddonName [Themes] set-default ThemeName install ThemeA, ThemeB [Settings] key:

Having a simple text-based config with a repo, like [AddOns] ensure-ffpkg AddonName [Themes] set-default ThemeName install ThemeA, ThemeB [Settings] key:val ... and have the AddonName downloaded with GNUPG signature check from an official Mozilla repo is by far more save that demand using interactive GUIs. Simply ask at startup to accept "potentially dangerous" extensions if you specify a local .xpi file it the same. What you describe is the classic Windows approach that have proved enough to be ineffective and only useful for commercial practice. Mozilla is formally a foundation and Firefox is formally a FOSS project...