diff --git a/toolkit/components/messaging-system/lib/SpecialMessageActions.jsm b/toolkit/components/messaging-system/lib/SpecialMessageActions.jsm
index 93b1dda27ecd..73a12f63e03c 100644
--- a/toolkit/components/messaging-system/lib/SpecialMessageActions.jsm
+++ b/toolkit/components/messaging-system/lib/SpecialMessageActions.jsm
@@ -104,6 +104,15 @@ const SpecialMessageActions = {
     window.getShellService().setAsDefault();
   },
 
+  /**
+   * Set browser as the default PDF handler.
+   *
+   * @param {Window} window Reference to a window object
+   */
+  setDefaultPDFHandler(window, onlyIfKnownBrowser = false) {
+    window.getShellService().setAsDefaultPDFHandler(onlyIfKnownBrowser);
+  },
+
   /**
    * Reset browser homepage and newtab to default with a certain section configuration
    *
@@ -305,6 +314,12 @@ const SpecialMessageActions = {
       case "SET_DEFAULT_BROWSER":
         this.setDefaultBrowser(window);
         break;
+      case "SET_DEFAULT_PDF_HANDLER":
+        this.setDefaultPDFHandler(
+          window,
+          action.data?.onlyIfKnownBrowser ?? false
+        );
+        break;
       case "PIN_CURRENT_TAB":
         let tab = window.gBrowser.selectedTab;
         window.gBrowser.pinTab(tab);
diff --git a/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/SpecialMessageActionSchemas.json b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/SpecialMessageActionSchemas.json
index fe35649f06bd..32e5f5ebb333 100644
--- a/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/SpecialMessageActionSchemas.json
+++ b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/SpecialMessageActionSchemas.json
@@ -367,6 +367,28 @@
           "additionalProperties": false,
           "description": "Message action to set Firefox as default browser"
         },
+        {
+          "type": "object",
+          "properties": {
+            "type": {
+              "type": "string",
+              "enum": ["SET_DEFAULT_PDF_HANDLER"]
+            },
+            "data": {
+              "type": "object",
+              "properties": {
+                "onlyIfKnownBrowser": {
+                  "type": "boolean",
+                  "description": "Only set Firefox as the default PDF handler if the current PDF handler is a known browser."
+                }
+              },
+              "additionalProperties": false
+            }
+          },
+          "required": ["type"],
+          "additionalProperties": false,
+          "description": "Message action to set Firefox as the default PDF handler"
+        },
         {
           "type": "object",
           "properties": {
diff --git a/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/index.md b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/index.md
index e3fcc6146db4..99a2e7692a98 100644
--- a/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/index.md
+++ b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/index.md
@@ -245,10 +245,23 @@ Action for pinning Firefox to the user's taskbar.
 
 ### `SET_DEFAULT_BROWSER`
 
-Action for configuring the default browser to Firefox on the user's system.
+Action for setting the default browser to Firefox on the user's system.
 
 - args: (none)
 
+### `SET_DEFAULT_PDF_HANDLER`
+
+Action for setting the default PDF handler to Firefox on the user's system.
+
+- args:
+```ts
+{
+  // Only set Firefox as the default PDF handler if the current PDF handler is a
+  // known browser.
+  onlyIfKnownBrowser?: boolean;
+}
+```
+
 ### `SHOW_SPOTLIGHT`
 
 Action for opening a spotlight tab or window modal using the content passed to the dialog.
diff --git a/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/test/browser/browser.ini b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/test/browser/browser.ini
index 8ffe0fc0cde8..b6118daab74d 100644
--- a/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/test/browser/browser.ini
+++ b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/test/browser/browser.ini
@@ -27,6 +27,7 @@ skip-if = os != "win"
 [browser_sma_cfrmessageprovider.js]
 [browser_sma_configure_homepage.js]
 [browser_sma_default_browser.js]
+[browser_sma_default_pdf_handler.js]
 [browser_sma_set_prefs.js]
 [browser_sma_click_element.js]
 [browser_sma_handle_multiaction.js]
diff --git a/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/test/browser/browser_sma_default_pdf_handler.js b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/test/browser/browser_sma_default_pdf_handler.js
new file mode 100644
index 000000000000..adb77b93e2e2
--- /dev/null
+++ b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/test/browser/browser_sma_default_pdf_handler.js
@@ -0,0 +1,92 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+add_task(async function test_set_default_pdf_handler_no_data() {
+  const sandbox = sinon.createSandbox();
+  const stub = sandbox.stub();
+
+  await SMATestUtils.executeAndValidateAction(
+    { type: "SET_DEFAULT_PDF_HANDLER" },
+    {
+      ownerGlobal: {
+        getShellService: () => ({
+          setAsDefaultPDFHandler: stub,
+        }),
+      },
+    }
+  );
+
+  Assert.equal(
+    stub.callCount,
+    1,
+    "setAsDefaultPDFHandler was called by the action"
+  );
+  Assert.ok(
+    stub.calledWithExactly(false),
+    "setAsDefaultPDFHandler called with onlyIfKnownBrowser = false"
+  );
+});
+
+add_task(async function test_set_default_pdf_handler_data_false() {
+  const sandbox = sinon.createSandbox();
+  const stub = sandbox.stub();
+
+  await SMATestUtils.executeAndValidateAction(
+    {
+      type: "SET_DEFAULT_PDF_HANDLER",
+      data: {
+        onlyIfKnownBrowser: false,
+      },
+    },
+    {
+      ownerGlobal: {
+        getShellService: () => ({
+          setAsDefaultPDFHandler: stub,
+        }),
+      },
+    }
+  );
+
+  Assert.equal(
+    stub.callCount,
+    1,
+    "setAsDefaultPDFHandler was called by the action"
+  );
+  Assert.ok(
+    stub.calledWithExactly(false),
+    "setAsDefaultPDFHandler called with onlyIfKnownBrowser = false"
+  );
+});
+
+add_task(async function test_set_default_pdf_handler_data_true() {
+  const sandbox = sinon.createSandbox();
+  const stub = sandbox.stub();
+
+  await SMATestUtils.executeAndValidateAction(
+    {
+      type: "SET_DEFAULT_PDF_HANDLER",
+      data: {
+        onlyIfKnownBrowser: true,
+      },
+    },
+    {
+      ownerGlobal: {
+        getShellService: () => ({
+          setAsDefaultPDFHandler: stub,
+        }),
+      },
+    }
+  );
+
+  Assert.equal(
+    stub.callCount,
+    1,
+    "setAsDefaultPDFHandler was called by the action"
+  );
+  Assert.ok(
+    stub.calledWithExactly(true),
+    "setAsDefaultPDFHandler called with onlyIfKnownBrowser = true"
+  );
+});