diff --git a/dom/security/nsCSPService.cpp b/dom/security/nsCSPService.cpp
index d30036c85c2f..16fb9fa9754f 100644
--- a/dom/security/nsCSPService.cpp
+++ b/dom/security/nsCSPService.cpp
@@ -172,11 +172,31 @@ bool subjectToCSP(nsIURI* aURI, nsContentPolicyType aContentType) {
   nsCOMPtr<nsIContentSecurityPolicy> csp = aLoadInfo->GetCsp();
 
   if (csp) {
+    // Generally aOriginalURI denotes the URI before a redirect and hence
+    // will always be a nullptr here. Only exception are frame navigations
+    // which we want to treat as a redirect for the purpose of CSP reporting
+    // and in particular the `blocked-uri` in the CSP report where we want
+    // to report the prePath information.
+    nsCOMPtr<nsIURI> originalURI = nullptr;
+    ExtContentPolicyType extType =
+        nsContentUtils::InternalContentPolicyTypeToExternal(contentType);
+    if (extType == ExtContentPolicy::TYPE_SUBDOCUMENT &&
+        !aLoadInfo->GetOriginalFrameSrcLoad() &&
+        mozilla::StaticPrefs::
+            security_csp_truncate_blocked_uri_for_frame_navigations()) {
+      nsAutoCString prePathStr;
+      nsresult rv = aContentLocation->GetPrePath(prePathStr);
+      NS_ENSURE_SUCCESS(rv, rv);
+      rv = NS_NewURI(getter_AddRefs(originalURI), prePathStr);
+      NS_ENSURE_SUCCESS(rv, rv);
+    }
+
     // obtain the enforcement decision
-    rv = csp->ShouldLoad(contentType, cspEventListener, aContentLocation,
-                         nullptr,  // no redirect, aOriginal URL is null.
-                         !isPreload && aLoadInfo->GetSendCSPViolationEvents(),
-                         cspNonce, parserCreatedScript, aDecision);
+    rv = csp->ShouldLoad(
+        contentType, cspEventListener, aContentLocation,
+        originalURI,  // no redirect, unless it's a frame navigation.
+        !isPreload && aLoadInfo->GetSendCSPViolationEvents(), cspNonce,
+        parserCreatedScript, aDecision);
 
     if (NS_CP_REJECTED(*aDecision)) {
       NS_SetRequestBlockingReason(
diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
index bea641a838d6..8c58252a8dc4 100644
--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -9475,6 +9475,11 @@
   value: 40
   mirror: always
 
+- name: security.csp.truncate_blocked_uri_for_frame_navigations
+  type: bool
+  value: true
+  mirror: always
+
 # Allows loading ui resources in CheckLoadURIFlags
 # TODO Bug 1654488: Remove pref in CheckLoadURIFlags
 #      which allows all UI resources to load
diff --git a/toolkit/components/extensions/test/xpcshell/test_ext_contentscript_triggeringPrincipal.js b/toolkit/components/extensions/test/xpcshell/test_ext_contentscript_triggeringPrincipal.js
index 223f0650e943..f5df8e61d2f5 100644
--- a/toolkit/components/extensions/test/xpcshell/test_ext_contentscript_triggeringPrincipal.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_contentscript_triggeringPrincipal.js
@@ -25,6 +25,12 @@ Services.prefs.setIntPref(
   4096
 );
 
+// Do not trunacate the blocked-uri in CSP reports for frame navigations.
+Services.prefs.setBoolPref(
+  "security.csp.truncate_blocked_uri_for_frame_navigations",
+  false
+);
+
 // ExtensionContent.jsm needs to know when it's running from xpcshell,
 // to use the right timeout for content scripts executed at document_idle.
 ExtensionTestUtils.mockAppInfo();
@@ -830,6 +836,8 @@ function computeBaseURLs(tests, expectedSources, forbiddenSources = {}) {
 
   function* iterSources(test, sources) {
     for (let [source, attrs] of Object.entries(sources)) {
+      // if a source defines attributes (e.g. liveSrc in PAGE_SOURCES etc.) then all
+      // attributes in the source must be matched by the test (see const TEST).
       if (Object.keys(attrs).every(attr => attrs[attr] === test[attr])) {
         yield `${BASE_URL}/${test.src}?source=${source}`;
       }
@@ -1082,6 +1090,9 @@ const TESTS = [
   },
   // TODO: <frame> element, which requires a frameset document.
   {
+    // the blocked-uri for frame-navigations is the pre-path URI. For the
+    // purpose of this test we do not strip the blocked-uri by setting the
+    // preference 'truncate_blocked_uri_for_frame_navigations'
     element: ["iframe", {}],
     src: "iframe.html",
   },