diff --git a/mozilla/modules/libjar/nsIJARURI.idl b/mozilla/modules/libjar/nsIJARURI.idl
index a2a84a06dcd..ed78aff8da6 100644
--- a/mozilla/modules/libjar/nsIJARURI.idl
+++ b/mozilla/modules/libjar/nsIJARURI.idl
@@ -51,14 +51,14 @@
 interface nsIJARURI : nsIURL {
 
     /**
-     * Returns the root URI (the one for the actual JAR file) for this JAR.
-     * eg http://www.big.com/blue.jar
+     * Returns the root URI (the one for the actual JAR file) for this JAR
+     * (e.g., http://www.big.com/blue.jar).
      */
     attribute nsIURI JARFile;
 
     /**
-     * Returns the entry specified for this JAR URI.
-     * eg ocean.html
+     * Returns the entry specified for this JAR URI (e.g., "ocean.html").  This
+     * value may contain %-escaped byte sequences.
      */
     attribute AUTF8String JAREntry;
 };
diff --git a/mozilla/modules/libjar/nsJARChannel.cpp b/mozilla/modules/libjar/nsJARChannel.cpp
index f3b1c882c15..7bba2b99092 100644
--- a/mozilla/modules/libjar/nsJARChannel.cpp
+++ b/mozilla/modules/libjar/nsJARChannel.cpp
@@ -41,6 +41,7 @@
 #include "nsMimeTypes.h"
 #include "nsNetUtil.h"
 #include "nsInt64.h"
+#include "nsEscape.h"
 
 #include "nsIScriptSecurityManager.h"
 #include "nsIPrincipal.h"
@@ -280,6 +281,12 @@ nsJARChannel::EnsureJarInput(PRBool blocking)
     rv = mJarURI->GetJAREntry(mJarEntry);
     if (NS_FAILED(rv)) return rv;
 
+    // The name of the JAR entry must not contains URL escaped characters:
+    // we're moving from URL domain to a filename domain here. nsStandardURL
+    // does basic escaping by default, which breaks reading zipped files which
+    // have e.g. spaces in their filenames.
+    NS_UnescapeURL(mJarEntry);
+
     // try to get a nsIFile directly from the url, which will often succeed.
     {
         nsCOMPtr<nsIFileURL> fileURL = do_QueryInterface(mJarBaseURI);