firefox: consider sideloading capability in extension handling

      # and an extid attribute

      extensions = if nameArray != (lib.unique nameArray) then

        throw "Firefox addon name needs to be unique"

      else if browser.requireSigning then

        throw "Nix addons are only supported with signature enforcement disabled"

      else if browser.requireSigning || !browser.allowAddonSideload then

        throw "Nix addons are only supported with signature enforcement disabled and addon sideloading enabled (eg. LibreWolf)"

      else builtins.map (a:

        if ! (builtins.hasAttr "extid" a) then

        throw "nixExtensions has an invalid entry. Missing extid attribute. Please use fetchfirefoxaddon"