diff --git a/remote/shared/webdriver/Capabilities.sys.mjs b/remote/shared/webdriver/Capabilities.sys.mjs
index ea978ba76a08..1a0dd4cbf977 100644
--- a/remote/shared/webdriver/Capabilities.sys.mjs
+++ b/remote/shared/webdriver/Capabilities.sys.mjs
@@ -47,7 +47,6 @@ export const WEBDRIVER_CLASSIC_CAPABILITIES = [
   "pageLoadStrategy",
   "strictFileInteractability",
   "timeouts",
-  "unhandledPromptBehavior",
   "webSocketUrl",
 
   // Gecko specific capabilities
diff --git a/remote/shared/webdriver/test/xpcshell/test_Session.js b/remote/shared/webdriver/test/xpcshell/test_Session.js
index 149479efc47e..bc86f3428df5 100644
--- a/remote/shared/webdriver/test/xpcshell/test_Session.js
+++ b/remote/shared/webdriver/test/xpcshell/test_Session.js
@@ -72,7 +72,7 @@ add_task(function test_WebDriverSession_ctor() {
   // BiDi session
   session = createSession({ capabilities, isBidi: true });
   equal(session.acceptInsecureCerts, true);
-  equal(session.userPromptHandler.toJSON(), "dismiss and notify");
+  equal(session.userPromptHandler.toJSON(), "ignore");
 
   equal(session.pageLoadStrategy, undefined);
   equal(session.strictFileInteractability, undefined);
diff --git a/remote/webdriver-bidi/test/browser/browser_WebDriverBiDi.js b/remote/webdriver-bidi/test/browser/browser_WebDriverBiDi.js
index 3dbc0e9ce769..008bdf1756c9 100644
--- a/remote/webdriver-bidi/test/browser/browser_WebDriverBiDi.js
+++ b/remote/webdriver-bidi/test/browser/browser_WebDriverBiDi.js
@@ -114,7 +114,7 @@ add_task(async function test_createSession() {
     wdBiDi => {
       is(wdBiDi.session.bidi, true);
       is(wdBiDi.session.acceptInsecureCerts, true);
-      is(wdBiDi.session.userPromptHandler.toJSON(), "dismiss and notify");
+      is(wdBiDi.session.userPromptHandler.toJSON(), "ignore");
 
       is(wdBiDi.session.pageLoadStrategy, undefined);
       is(wdBiDi.session.strictFileInteractability, undefined);
diff --git a/testing/web-platform/mozilla/tests/webdriver/bidi/session/new/capabilities.py b/testing/web-platform/mozilla/tests/webdriver/bidi/session/new/capabilities.py
index a63afc8d7ac4..80ab5820b732 100644
--- a/testing/web-platform/mozilla/tests/webdriver/bidi/session/new/capabilities.py
+++ b/testing/web-platform/mozilla/tests/webdriver/bidi/session/new/capabilities.py
@@ -49,6 +49,47 @@ async def test_proxy(
     assert response == {"type": "string", "value": page_content}
 
 
+@pytest.mark.parametrize(
+    "value",
+    [
+        None,
+        "accept",
+        "accept and notify",
+        "dismiss",
+        "dismiss and notify",
+        "ignore",
+    ],
+)
+async def test_unhandledPromptBehavior_string(new_session, match_capabilities, value):
+    capabilities = match_capabilities("alwaysMatch", "unhandledPromptBehavior", value)
+
+    bidi_session = await new_session(capabilities=capabilities)
+
+    if value is None:
+        assert (
+            bidi_session.capabilities.get("unhandledPromptBehavior")
+            == "dismiss and notify"
+        )
+    else:
+        assert bidi_session.capabilities.get("unhandledPromptBehavior") == value
+
+
+@pytest.mark.parametrize("handler", ["accept", "dismiss", "ignore"])
+@pytest.mark.parametrize("prompt_type", ["alert", "beforeUnload", "confirm", "prompt"])
+async def test_unhandledPromptBehavior_object(
+    new_session, match_capabilities, handler, prompt_type
+):
+    capabilities = match_capabilities(
+        "alwaysMatch", "unhandledPromptBehavior", {prompt_type: handler}
+    )
+
+    bidi_session = await new_session(capabilities=capabilities)
+
+    assert bidi_session.capabilities.get("unhandledPromptBehavior") == {
+        prompt_type: handler
+    }
+
+
 @pytest.mark.parametrize("match_type", ["alwaysMatch", "firstMatch"])
 async def test_websocket_url(new_session, match_capabilities, match_type):
     capabilities = match_capabilities(match_type, "webSocketUrl", True)