Fix dislike count increasing in Firefox.

likeClicked() suppose to decrease the dislike count by 1 if the
previous state is "disliked", but the function is empty on Firefox.

Extensions/firefox/return-youtube-dislike.script.js

@@ -116,7 +116,13 @@ function setState() {
   );
 }
 
-function likeClicked() {}
+function likeClicked() {
+  if (storedData.previousState === 'disliked') {
+    storedData.dislikes--;
+    setDislikes(numberFormat(storedData.dislikes));
+    storedData.previousState = 'liked';
+  }
+}
 
 function dislikeClicked() {
   let state = getState().current;