diff --git a/dom/interfaces/security/nsIContentSecurityPolicy.idl b/dom/interfaces/security/nsIContentSecurityPolicy.idl
index 7c1454a54473..ee737d563f06 100644
--- a/dom/interfaces/security/nsIContentSecurityPolicy.idl
+++ b/dom/interfaces/security/nsIContentSecurityPolicy.idl
@@ -329,7 +329,6 @@ interface nsIContentSecurityPolicy : nsISerializable
   short shouldLoad(in nsContentPolicyType aContentType,
                    in nsICSPEventListener aCSPEventListener,
                    in nsIURI          aContentLocation,
-                   in ACString        aMimeTypeGuess,
                    in nsIURI          aOriginalURIIfRedirect,
                    in bool            aSendViolationReports,
                    in AString         aNonce,
diff --git a/dom/security/nsCSPContext.cpp b/dom/security/nsCSPContext.cpp
index b94b76a22b46..7eb37179a938 100644
--- a/dom/security/nsCSPContext.cpp
+++ b/dom/security/nsCSPContext.cpp
@@ -118,7 +118,6 @@ NS_IMETHODIMP
 nsCSPContext::ShouldLoad(nsContentPolicyType aContentType,
                          nsICSPEventListener* aCSPEventListener,
                          nsIURI* aContentLocation,
-                         const nsACString& aMimeTypeGuess,
                          nsIURI* aOriginalURIIfRedirect,
                          bool aSendViolationReports, const nsAString& aNonce,
                          bool aParserCreated, int16_t* outDecision) {
diff --git a/dom/security/nsCSPService.cpp b/dom/security/nsCSPService.cpp
index 32c2c1f4518d..70ecf56af4b0 100644
--- a/dom/security/nsCSPService.cpp
+++ b/dom/security/nsCSPService.cpp
@@ -151,7 +151,7 @@ bool subjectToCSP(nsIURI* aURI, nsContentPolicyType aContentType) {
     if (preloadCsp) {
       // obtain the enforcement decision
       rv = preloadCsp->ShouldLoad(
-          contentType, cspEventListener, aContentLocation, aMimeTypeGuess,
+          contentType, cspEventListener, aContentLocation,
           nullptr,  // no redirect, aOriginal URL is null.
           aLoadInfo->GetSendCSPViolationEvents(), cspNonce, parserCreatedScript,
           aDecision);
@@ -177,7 +177,6 @@ bool subjectToCSP(nsIURI* aURI, nsContentPolicyType aContentType) {
   if (csp) {
     // obtain the enforcement decision
     rv = csp->ShouldLoad(contentType, cspEventListener, aContentLocation,
-                         aMimeTypeGuess,
                          nullptr,  // no redirect, aOriginal URL is null.
                          aLoadInfo->GetSendCSPViolationEvents(), cspNonce,
                          parserCreatedScript, aDecision);
@@ -361,7 +360,6 @@ nsresult CSPService::ConsultCSPForRedirect(nsIURI* aOriginalURI,
           policyType,  // load type per nsIContentPolicy (uint32_t)
           cspEventListener,
           aNewURI,       // nsIURI
-          ""_ns,         // ACString - MIME guess
           aOriginalURI,  // Original nsIURI
           true,          // aSendViolationReports
           cspNonce,      // nonce
@@ -383,7 +381,6 @@ nsresult CSPService::ConsultCSPForRedirect(nsIURI* aOriginalURI,
     csp->ShouldLoad(policyType,  // load type per nsIContentPolicy (uint32_t)
                     cspEventListener,
                     aNewURI,       // nsIURI
-                    ""_ns,         // ACString - MIME guess
                     aOriginalURI,  // Original nsIURI
                     true,          // aSendViolationReports
                     cspNonce,      // nonce
diff --git a/dom/security/test/unit/test_csp_reports.js b/dom/security/test/unit/test_csp_reports.js
index f1341f3588f7..935a38c28ccc 100644
--- a/dom/security/test/unit/test_csp_reports.js
+++ b/dom/security/test/unit/test_csp_reports.js
@@ -182,7 +182,6 @@ function run_test() {
       null, // nsICSPEventListener
       NetUtil.newURI("http://blocked.test/foo.js"),
       null,
-      null,
       true,
       null,
       false
@@ -243,7 +242,6 @@ function run_test() {
       null, // nsICSPEventListener
       NetUtil.newURI("data:image/png;base64," + base64data),
       null,
-      null,
       true,
       null,
       false
@@ -258,7 +256,6 @@ function run_test() {
       null, // nsICSPEventListener
       NetUtil.newURI("intent://mymaps.com/maps?um=1&ie=UTF-8&fb=1&sll"),
       null,
-      null,
       true,
       null,
       false
@@ -275,7 +272,6 @@ function run_test() {
       null, // nsICSPEventListener
       NetUtil.newURI(selfSpec + "#bar"),
       null,
-      null,
       true,
       null,
       false
@@ -294,7 +290,6 @@ function run_test() {
         null, // nsICSPEventListener
         NetUtil.newURI("ftp://blocked.test/profile.png"),
         null,
-        null,
         true,
         null,
         false