firefox: prepare for updated sideloading behavior

Co-Authored-By: Cole Helbling <cole.e.helbling@outlook.com>

modules/misc/news.nix

@@ -1381,6 +1381,32 @@ in
           A new module is available: 'services.lieer'.
         '';
       }
+
+      {
+        time = "2020-03-15T16:55:28+00:00";
+        condition = config.programs.firefox.enable;
+        message = ''
+          In anticipation of Firefox dropping support for extension
+          sideloading[1], we now install extensions directly to
+          Firefox profiles managed through Home Manager's
+
+            'programs.firefox.profiles'
+
+          option.
+
+          Unfortunately this will most likely trigger an "Existing
+          file is in the way" error when activating your configuration
+          since Firefox keeps a copy of the add-on in the location
+          Home Manager wants to overwrite. If this is the case, remove
+          the listed '.xpi' files and try again.
+
+          This change also means that extensions installed through
+          Home Manager may disappear from unmanaged profiles in future
+          Firefox releases.
+
+          [1] https://blog.mozilla.org/addons/2019/10/31/firefox-to-discontinue-sideloaded-extensions/
+        '';
+      }
     ];
   };
 }

modules/programs/firefox.nix

@@ -23,8 +23,15 @@ let
     then "${firefoxConfigPath}/Profiles"
     else firefoxConfigPath;
 
+  # The extensions path shared by all profiles; will not be supported
+  # by future Firefox versions.
   extensionPath = "extensions/{ec8030f7-c20a-464f-9b0e-13a3a9e97384}";
 
+  extensionsEnvPkg = pkgs.buildEnv {
+    name = "hm-firefox-extensions";
+    paths = cfg.extensions;
+  };
+
   profiles =
     flip mapAttrs' cfg.profiles (_: profile:
       nameValuePair "Profile${toString profile.id}" {
@@ -283,17 +290,10 @@ in
 
     home.file = mkMerge (
       [{
-        "${mozillaConfigPath}/${extensionPath}" = mkIf (cfg.extensions != []) (
+        "${mozillaConfigPath}/${extensionPath}" = mkIf (cfg.extensions != []) {
-          let
+          source = "${extensionsEnvPkg}/share/mozilla/${extensionPath}";
-            extensionsEnv = pkgs.buildEnv {
+          recursive = true;
-              name = "hm-firefox-extensions";
+        };
-              paths = cfg.extensions;
-            };
-          in {
-            source = "${extensionsEnv}/share/mozilla/${extensionPath}";
-            recursive = true;
-          }
-        );
 
         "${firefoxConfigPath}/profiles.ini" = mkIf (cfg.profiles != {}) {
           text = profilesIni;
@@ -314,6 +314,11 @@ in
           mkIf (profile.settings != {} || profile.extraConfig != "") {
             text = mkUserJs profile.settings profile.extraConfig;
           };
+
+        "${profilesPath}/${profile.path}/extensions" = mkIf (cfg.extensions != []) {
+          source = "${extensionsEnvPkg}/share/mozilla/${extensionPath}";
+          recursive = true;
+        };
       })
     );
   };