import changes from tenfourfox:

- #375, Bug 1112537 - Optimize String#split('foo').join('bar') pattern. (includes M1235403) r=nbp (36326dfa2)
- #415: M1373195 backport (e880223af)
- #415: M1376936 M1375331 (33344c644)
- #415: M1348095 (consolidated) (d1001e7ef)
- #413: update to HTML colspan/rowspan from M1359822, plus M1271126 + M1373095 (no tests) (6958789e8)
- #416: M1366903 M1371283 M1368576 (baaf2ee89)
- #416: M1368105 (1efa1540f)
- checkin additional JS tests (687ba7579)
- #416: M1374047 M1365413 M1369913 M1371424 M1346590 M1376087 M1322896 M1354796 M1365333 M1373970 M1355168 (1bc1dd390)
- #416: M1265568 M1305036 (257cfd929)
- #419: M1377329 (fb72079d0)
- #418: M1333106 M1337672 (b1cfd7830)
- #416: M1362924 (61ec43889)
- #416: M1365189 M1365875 (d9876efcf)
- #416: M1378147 M1371890 M1380426 (177c87d34)
- #416: note to self on M1377426 (5bcd1c461)
- fix debug bustage, shutdown crash (be4d0ed86)
- #414: new wildcard based blacklist, updated strings (6ccb443e7)
- #416: update certs/pins (797260262)
- fix FontDirWrapper off-by-one bugs and warnings (51e6a2cf9)
- 104fx_copy should also allow copying debug builds (054bbf081)
- #416: M1383000 M1376459 M1372467 M1372383 M1383002 (2f4742e52)
- #416: M1308908 M1342433(no tests) (5757a9534)
- #416: M1368362 (1cf170047)
- #72: additional debugging (0683827d2)
- #416: M1381016 M1371657 (afc820240)
- #416: update certs for FPR2 final (4ca64d74e)
- M1257208 Fix build errors with gcc >= 5 (4c64d8d6d)
with additional nsINode* to nsTextNode* change in ResetDir() in DirectionalityUtils.cpp

uriloader/prefetch/nsPrefetchService.cpp

@@ -14,6 +14,7 @@
 #include "nsIHttpChannel.h"
 #include "nsIURL.h"
 #include "nsISimpleEnumerator.h"
+#include "nsISupportsPriority.h"
 #include "nsNetUtil.h"
 #include "nsString.h"
 #include "nsXPIDLString.h"
@@ -134,7 +135,19 @@ nsPrefetchNode::OpenChannel()
             false);
     }
 
-    return mChannel->AsyncOpen2(this);
+    // Reduce the priority of prefetch network requests.
+    nsCOMPtr<nsISupportsPriority> priorityChannel = do_QueryInterface(mChannel);
+    if (priorityChannel) {
+      priorityChannel->AdjustPriority(nsISupportsPriority::PRIORITY_LOWEST);
+    }
+
+    rv = mChannel->AsyncOpen2(this);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      // Drop the ref to the channel, because we don't want to end up with
+      // cycles through it.
+      mChannel = nullptr;
+    }
+    return rv;
 }
 
 nsresult