diff --git a/security/manager/ssl/nsISiteSecurityService.idl b/security/manager/ssl/nsISiteSecurityService.idl
index 753f32b57..b61577152 100644
--- a/security/manager/ssl/nsISiteSecurityService.idl
+++ b/security/manager/ssl/nsISiteSecurityService.idl
@@ -23,7 +23,7 @@ namespace mozilla
 [ref] native nsCStringTArrayRef(nsTArray<nsCString>);
 [ref] native mozillaPkixTime(mozilla::pkix::Time);
 
-[scriptable, uuid(275127f8-dbd7-4681-afbf-6df0c6587a01)]
+[scriptable, uuid(233908bd-6741-4474-a6e1-f298c6ce9eaf)]
 interface nsISiteSecurityService : nsISupports
 {
     const uint32_t HEADER_HSTS = 0;
@@ -98,15 +98,21 @@ interface nsISiteSecurityService : nsISupports
      * Given a header type, removes state relating to that header of a host,
      * including the includeSubdomains state that would affect subdomains.
      * This essentially removes the state for the domain tree rooted at this
-     * host.
+     * host. If any preloaded information is present for that host, that
+     * information will then be used instead of any other previously existing
+     * state, unless the force parameter is set.
+     *
      * @param aType   the type of security state in question
      * @param aURI    the URI of the target host
      * @param aFlags  options for this request as defined in nsISocketProvider:
      *                  NO_PERMANENT_STORAGE
+     * @param force   if set, forces no-HSTS state by writing a knockout value,
+     *                overriding any preload list state
      */
     void removeState(in uint32_t aType,
                      in nsIURI aURI,
-                     in uint32_t aFlags);
+                     in uint32_t aFlags,
+                     [optional] in boolean force);
 
     /**
      * See isSecureURI
diff --git a/security/manager/ssl/nsSiteSecurityService.cpp b/security/manager/ssl/nsSiteSecurityService.cpp
index cfee79d8d..44ee7dcc0 100644
--- a/security/manager/ssl/nsSiteSecurityService.cpp
+++ b/security/manager/ssl/nsSiteSecurityService.cpp
@@ -330,21 +330,22 @@ nsSiteSecurityService::SetHSTSState(uint32_t aType,
                                     uint32_t flags,
                                     SecurityPropertyState aHSTSState)
 {
-  // If max-age is zero, that's an indication to immediately remove the
-  // security state, so here's a shortcut.
-  if (!maxage) {
-    return RemoveState(aType, aSourceURI, flags);
+  // Exit early if STS not enabled
+  if (!mUseStsService) {
+    return NS_OK;
+  }
+
+  // If max-age is zero, the host is no longer considered HSTS. If the host was
+  // preloaded, we store an entry indicating that this host is not HSTS, causing
+  // the preloaded information to be ignored.
+  if (maxage == 0) {
+    return RemoveState(aType, aSourceURI, flags, true);
   }
 
   MOZ_ASSERT((aHSTSState == SecurityPropertySet ||
               aHSTSState == SecurityPropertyNegative),
       "HSTS State must be SecurityPropertySet or SecurityPropertyNegative");
 
-  // Exit early if STS not enabled
-  if (!mUseStsService) {
-    return NS_OK;
-  }
-
   int64_t expiretime = ExpireTimeFromMaxAge(maxage);
   SiteHSTSState siteState(expiretime, aHSTSState, includeSubdomains);
   nsAutoCString stateString;
@@ -367,7 +368,7 @@ nsSiteSecurityService::SetHSTSState(uint32_t aType,
 
 NS_IMETHODIMP
 nsSiteSecurityService::RemoveState(uint32_t aType, nsIURI* aURI,
-                                   uint32_t aFlags)
+                                   uint32_t aFlags, bool force = false)
 {
    // Child processes are not allowed direct access to this.
    if (!XRE_IsParentProcess()) {
@@ -387,8 +388,9 @@ nsSiteSecurityService::RemoveState(uint32_t aType, nsIURI* aURI,
   mozilla::DataStorageType storageType = isPrivate
                                          ? mozilla::DataStorage_Private
                                          : mozilla::DataStorage_Persistent;
-  // If this host is in the preload list, we have to store a knockout entry.
-  if (GetPreloadListEntry(hostname.get())) {
+  // If this host is in the preload list, we have to store a knockout entry
+  // if it's explicitly forced to not be HSTS anymore
+  if (force && GetPreloadListEntry(hostname.get())) {
     SSSLOG(("SSS: storing knockout entry for %s", hostname.get()));
     SiteHSTSState siteState(0, SecurityPropertyKnockout, false);
     nsAutoCString stateString;
@@ -769,7 +771,10 @@ nsSiteSecurityService::ProcessPKPHeader(nsIURI* aSourceURI,
     return NS_ERROR_FAILURE;
   }
 
-  // if maxAge == 0 we must delete all state, for now no hole-punching
+  // If maxAge == 0, we remove dynamic HPKP state for this host. Due to
+  // architectural constraints, if this host was preloaded, any future lookups
+  // will use the preloaded state (i.e. we can't store a "this host is not HPKP"
+  // entry like we can for HSTS).
   if (maxAge == 0) {
     return RemoveState(aType, aSourceURI, aFlags);
   }