progs: firefox: *.userChrome may be path or drv

This allows `programs.firefox.profiles.*.userChrome` to be set to a:
derivation, path/path-like string to directory or file, or multiline
text to be used as content verbatim.

This allows setting, for example(s):
```nix
programs.firefox.profiles."jacob.default".userChrome = pkgs.wavefox;
programs.firefox.profiles."jacob.default".userChrome =
    "${pkgs.wavefox}/userChrome.css";
```

Author: spikespaz

modules/programs/firefox/mkFirefoxModule.nix

@@ -358,7 +358,12 @@ in {
           userChrome = mkOption {
             type = types.oneOf [ types.lines types.path ];
             default = "";
-            description = "Custom ${appName} user chrome CSS.";
+            description = ''
+              Custom ${appName} user chrome CSS.
+
+              This can be a path to a file or directory in the nix store,
+              or a derivation, or a verbatim multi-line string.
+            '';
             example = ''
               /* Hide tab bar in FF Quantum */
               @-moz-document url(chrome://browser/content/browser.xul), url(chrome://browser/content/browser.xhtml) {
@@ -754,66 +759,78 @@ in {
       "${cfg.configPath}/profiles.ini" =
         mkIf (cfg.profiles != { }) { text = profilesIni; };
     }] ++ lib.flip mapAttrsToList cfg.profiles (_: profile:
-      # Merge the regular profile settings with extension settings
-      mkMerge ([{
-        "${profilesPath}/${profile.path}/.keep".text = "";
-
-        "${profilesPath}/${profile.path}/chrome/userChrome.css" =
-          mkIf (profile.userChrome != "") (let
-            key =
-              if builtins.isString profile.userChrome then "text" else "source";
-          in { "${key}" = profile.userChrome; });
-
-        "${profilesPath}/${profile.path}/chrome/userContent.css" =
-          mkIf (profile.userContent != "") (let
-            key = if builtins.isString profile.userContent then
-              "text"
-            else
-              "source";
-          in { "${key}" = profile.userContent; });
-
-        "${profilesPath}/${profile.path}/user.js" = mkIf (profile.preConfig
-          != "" || profile.settings != { } || profile.extraConfig != ""
-          || profile.bookmarks.configFile != null) {
-            text =
-              mkUserJs profile.preConfig profile.settings profile.extraConfig
-              profile.bookmarks.configFile profile.extensions.settings;
-          };
+      let
+        chromePath = if lib.pathIsDirectory profile.userChrome then
+          "chrome"
+        else
+          "chrome/userChrome.css";
+        sourcePath = if lib.types.path.check profile.userChrome then
+          profile.userChrome
+        else
+          null;
+        # Merge the regular profile settings with extension settings
+      in mkMerge [
+        {
+          "${profilesPath}/${profile.path}/.keep".text = "";
+
+          "${profilesPath}/${profile.path}/${chromePath}" =
+            if sourcePath == null then {
+              text = profile.userChrome;
+            } else {
+              source = sourcePath;
+            };
 
-        "${profilesPath}/${profile.path}/containers.json" =
-          mkIf (profile.containers != { }) {
-            text = mkContainersJson profile.containers;
-            force = profile.containersForce;
-          };
+          "${profilesPath}/${profile.path}/chrome/userContent.css" =
+            mkIf (profile.userContent != "") (let
+              key = if builtins.isString profile.userContent then
+                "text"
+              else
+                "source";
+            in { "${key}" = profile.userContent; });
+
+          "${profilesPath}/${profile.path}/user.js" = mkIf (profile.preConfig
+            != "" || profile.settings != { } || profile.extraConfig != ""
+            || profile.bookmarks.configFile != null) {
+              text =
+                mkUserJs profile.preConfig profile.settings profile.extraConfig
+                profile.bookmarks.configFile profile.extensions.settings;
+            };
 
-        "${profilesPath}/${profile.path}/search.json.mozlz4" =
-          mkIf (profile.search.enable) {
-            enable = profile.search.enable;
-            force = profile.search.force;
-            source = profile.search.file;
-          };
+          "${profilesPath}/${profile.path}/containers.json" =
+            mkIf (profile.containers != { }) {
+              text = mkContainersJson profile.containers;
+              force = profile.containersForce;
+            };
 
-        "${profilesPath}/${profile.path}/extensions" =
-          mkIf (profile.extensions.packages != [ ]) {
-            source = let
-              extensionsEnvPkg = pkgs.buildEnv {
-                name = "hm-firefox-extensions";
-                paths = profile.extensions.packages;
-              };
-            in "${extensionsEnvPkg}/share/mozilla/${extensionPath}";
-            recursive = true;
-            force = true;
-          };
-      }] ++
+          "${profilesPath}/${profile.path}/search.json.mozlz4" =
+            mkIf (profile.search.enable) {
+              enable = profile.search.enable;
+              force = profile.search.force;
+              source = profile.search.file;
+            };
+
+          "${profilesPath}/${profile.path}/extensions" =
+            mkIf (profile.extensions.packages != [ ]) {
+              source = let
+                extensionsEnvPkg = pkgs.buildEnv {
+                  name = "hm-firefox-extensions";
+                  paths = profile.extensions.packages;
+                };
+              in "${extensionsEnvPkg}/share/mozilla/${extensionPath}";
+              recursive = true;
+              force = true;
+            };
+        }
         # Add extension settings as separate attributes
-        optional (profile.extensions.settings != { }) (mkMerge (mapAttrsToList
+        (mkIf (profile.extensions.settings != { }) (mkMerge (mapAttrsToList
           (name: settingConfig: {
             "${profilesPath}/${profile.path}/browser-extension-data/${name}/storage.js" =
               {
                 force = settingConfig.force || profile.extensions.force;
                 text = lib.generators.toJSON { } settingConfig.settings;
               };
-          }) profile.extensions.settings)))));
+          }) profile.extensions.settings)))
+      ]));
   } // setAttrByPath modulePath {
     finalPackage = wrapPackage cfg.package;