diff --git a/firefox.spec b/firefox.spec
index b003eb0..9c50e23 100644
--- a/firefox.spec
+++ b/firefox.spec
@@ -63,7 +63,8 @@ Patch204:	rhbz-966424.patch
 Patch215:	firefox-enable-addons.patch
 Patch217:	firefox-baseline-disable.patch
 # Upstream patches
-Patch300:        mozilla-858919.patch
+Patch300:	mozilla-858919.patch
+Patch301:	mozilla-1097550-dict-fix.patch
 
 BuildRequires:	gtk+2-devel
 BuildRequires:	unzip
@@ -190,6 +191,7 @@ Files and macros mainly for building Firefox extensions.
 
 # Upstream patches
 %patch300 -p1 -b .858919
+%patch301 -p1 -b .1097550-dict-fix
 
 pushd js/src
 autoconf-2.13
diff --git a/mozilla-1097550-dict-fix.patch b/mozilla-1097550-dict-fix.patch
new file mode 100644
index 0000000..9237247
--- /dev/null
+++ b/mozilla-1097550-dict-fix.patch
@@ -0,0 +1,72 @@
+diff -up mozilla-release/editor/composer/nsEditorSpellCheck.cpp.1097550-dict-fix mozilla-release/editor/composer/nsEditorSpellCheck.cpp
+--- mozilla-release/editor/composer/nsEditorSpellCheck.cpp.1097550-dict-fix	2014-11-26 03:17:14.000000000 +0100
++++ mozilla-release/editor/composer/nsEditorSpellCheck.cpp	2014-12-04 16:45:07.192923283 +0100
+@@ -103,6 +103,23 @@ GetLoadContext(nsIEditor* aEditor)
+ }
+ 
+ /**
++ * Helper function for converting underscore to dash in dictionary name,
++ * ie. en_CA to en-CA. This is required for some Linux distributions which
++ * use underscore as separator in system-wide installed dictionaries.
++ * We use it for nsStyleUtil::DashMatchCompare.
++ */
++static nsString
++GetDictNameWithDash(const nsAString& aDictName)
++{
++  nsString dictNameWithDash(aDictName);
++  int32_t underScore = dictNameWithDash.FindChar('_');
++  if (underScore != -1) {
++    dictNameWithDash.Replace(underScore, 1, '-');
++  }
++  return dictNameWithDash;
++}
++
++/**
+  * Fetches the dictionary stored in content prefs and maintains state during the
+  * fetch, which is asynchronous.
+  */
+@@ -606,8 +623,8 @@ nsEditorSpellCheck::SetCurrentDictionary
+     } else {
+       langCode.Assign(aDictionary);
+     }
+-
+-    if (mPreferredLang.IsEmpty() || !nsStyleUtil::DashMatchCompare(mPreferredLang, langCode, comparator)) {
++    if (mPreferredLang.IsEmpty() ||
++        !nsStyleUtil::DashMatchCompare(GetDictNameWithDash(mPreferredLang), langCode, comparator)) {
+       // When user sets dictionary manually, we store this value associated
+       // with editor url.
+       StoreCurrentDictionary(mEditor, aDictionary);
+@@ -763,12 +780,6 @@ nsEditorSpellCheck::DictionaryFetched(Di
+ 
+   // otherwise, get language from preferences
+   nsAutoString preferedDict(Preferences::GetLocalizedString("spellchecker.dictionary"));
+-  // Replace '_' with '-' in case the user has an underscore stored in their
+-  // pref, see bug 992118 for how this could have happened.
+-  int32_t underScore = preferedDict.FindChar('_');
+-  if (underScore != -1) {
+-    preferedDict.Replace(underScore, 1, '-');
+-  }
+   if (dictName.IsEmpty()) {
+     dictName.Assign(preferedDict);
+   }
+@@ -807,8 +818,8 @@ nsEditorSpellCheck::DictionaryFetched(Di
+ 
+       // try dictionary.spellchecker preference if it starts with langCode (and
+       // if we haven't tried it already)
+-      if (!preferedDict.IsEmpty() && !dictName.Equals(preferedDict) && 
+-          nsStyleUtil::DashMatchCompare(preferedDict, langCode, comparator)) {
++      if (!preferedDict.IsEmpty() && !dictName.Equals(preferedDict) &&
++          nsStyleUtil::DashMatchCompare(GetDictNameWithDash(preferedDict), langCode, comparator)) {
+         rv = SetCurrentDictionary(preferedDict);
+       }
+ 
+@@ -836,8 +847,7 @@ nsEditorSpellCheck::DictionaryFetched(Di
+             // We have already tried it
+             continue;
+           }
+-
+-          if (nsStyleUtil::DashMatchCompare(dictStr, langCode, comparator) &&
++          if (nsStyleUtil::DashMatchCompare(GetDictNameWithDash(dictStr), langCode, comparator) &&
+               NS_SUCCEEDED(SetCurrentDictionary(dictStr))) {
+               break;
+           }