diff --git a/modules/programs/firefox.nix b/modules/programs/firefox.nix
index e1594a946..c43781a7a 100644
--- a/modules/programs/firefox.nix
+++ b/modules/programs/firefox.nix
@@ -8,6 +8,32 @@ let
 
   extensionPath = "extensions/{ec8030f7-c20a-464f-9b0e-13a3a9e97384}";
 
+  profiles =
+    flip mapAttrs' cfg.profiles (_: profile:
+      nameValuePair "Profile${toString profile.id}" {
+        Name = profile.name;
+        Path = profile.path;
+        IsRelative = 1;
+        Default = if profile.isDefault then 1 else 0;
+      }
+    ) // {
+      General = {
+        StartWithLastProfile = 1;
+      };
+    };
+
+  profilesIni = generators.toINI {} profiles;
+
+  mkUserJs = prefs: extraPrefs: ''
+    // Generated by Home Manager.
+
+    ${concatStrings (mapAttrsToList (name: value: ''
+      user_pref("${name}", ${builtins.toJSON value});
+    '') prefs)}
+
+    ${extraPrefs}
+  '';
+
 in
 
 {
@@ -40,6 +66,84 @@ in
         '';
       };
 
+      profiles = mkOption {
+        type = types.attrsOf (types.submodule ({config, name, ...}: {
+          options = {
+            name = mkOption {
+              type = types.str;
+              default = name;
+              description = "Profile name.";
+            };
+
+            id = mkOption {
+              type = types.ints.unsigned;
+              default = 0;
+              description = ''
+                Profile ID. This should be set to a unique number per profile.
+              '';
+            };
+
+            settings = mkOption {
+              type = with types; attrsOf (either bool (either int str));
+              default = {};
+              example = literalExample ''
+                {
+                  "browser.startup.homepage" = "https://nixos.org";
+                  "browser.search.region" = "GB";
+                  "browser.search.isUS" = false;
+                  "distribution.searchplugins.defaultLocale" = "en-GB";
+                  "general.useragent.locale" = "en-GB";
+                  "browser.bookmarks.showMobileBookmarks" = true;
+                }
+              '';
+              description = "Attribute set of Firefox preferences.";
+            };
+
+            extraConfig = mkOption {
+              type = types.lines;
+              default = "";
+              description = ''
+                Extra preferences to add to <filename>user.js</filename>.
+              '';
+            };
+
+            userChrome = mkOption {
+              type = types.lines;
+              default = "";
+              description = "Custom Firefox CSS.";
+              example = ''
+                /* Hide tab bar in FF Quantum */
+                @-moz-document url("chrome://browser/content/browser.xul") {
+                  #TabsToolbar {
+                    visibility: collapse !important;
+                    margin-bottom: 21px !important;
+                  }
+
+                  #sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
+                    visibility: collapse !important;
+                  }
+                }
+              '';
+            };
+
+            path = mkOption {
+              type = types.str;
+              default = name;
+              description = "Profile path.";
+            };
+
+            isDefault = mkOption {
+              type = types.bool;
+              default = config.id == 0;
+              defaultText = "true if profile ID is 0";
+              description = "Whether this is a default profile.";
+            };
+          };
+        }));
+        default = {};
+        description = "Attribute set of Firefox profiles.";
+      };
+
       enableAdobeFlash = mkOption {
         type = types.bool;
         default = false;
@@ -81,6 +185,39 @@ in
   };
 
   config = mkIf cfg.enable {
+    assertions = [
+      (
+        let
+          defaults =
+            catAttrs "name" (filter (a: a.isDefault) (attrValues cfg.profiles));
+        in {
+          assertion = cfg.profiles == {} || length defaults == 1;
+          message =
+            "Must have exactly one default Firefox profile but found "
+            + toString (length defaults)
+            + optionalString (length defaults > 1)
+                (", namely " + concatStringsSep ", " defaults);
+        }
+      )
+
+      (
+        let
+          duplicates =
+            filterAttrs (_: v: length v != 1)
+            (zipAttrs
+            (mapAttrsToList (n: v: { "${toString v.id}" = n; })
+            (cfg.profiles)));
+
+          mkMsg = n: v: "  - ID ${n} is used by ${concatStringsSep ", " v}";
+        in {
+          assertion = duplicates == {};
+          message =
+            "Must not have Firefox profiles with duplicate IDs but\n"
+            + concatStringsSep "\n" (mapAttrsToList mkMsg duplicates);
+        }
+      )
+    ];
+
     home.packages =
       let
         # A bit of hackery to force a config into the wrapper.
@@ -99,17 +236,35 @@ in
       in
         [ (wrapper cfg.package { }) ];
 
-    home.file.".mozilla/${extensionPath}" = mkIf (cfg.extensions != []) (
-      let
-        extensionsEnv = pkgs.buildEnv {
-          name = "hm-firefox-extensions";
-          paths = cfg.extensions;
+    home.file = mkMerge (
+      [{
+        ".mozilla/${extensionPath}" = mkIf (cfg.extensions != []) (
+          let
+            extensionsEnv = pkgs.buildEnv {
+              name = "hm-firefox-extensions";
+              paths = cfg.extensions;
+            };
+          in {
+            source = "${extensionsEnv}/share/mozilla/${extensionPath}";
+            recursive = true;
+          }
+        );
+
+        ".mozilla/firefox/profiles.ini" = mkIf (cfg.profiles != {}) {
+          text = profilesIni;
         };
-      in
-        {
-          source = "${extensionsEnv}/share/mozilla/${extensionPath}";
-          recursive = true;
-        }
+      }]
+      ++ flip mapAttrsToList cfg.profiles (_: profile: {
+        ".mozilla/firefox/${profile.path}/chrome/userChrome.css" =
+          mkIf (profile.userChrome != "") {
+            text = profile.userChrome;
+          };
+
+        ".mozilla/firefox/${profile.path}/user.js" =
+          mkIf (profile.settings != {} || profile.extraConfig != "") {
+            text = mkUserJs profile.settings profile.extraConfig;
+          };
+      })
     );
   };
 }