Hi I believe that the "undo" feature is designed for a quick undo of a hasty mistake and may not undo such a mistake after a period of time. I have had a look and you can download the Wikipedia panel for Firefox for Android from [https://addons.mozilla.org/en-us/android/addon/wikipedia-panel/?src=hp-dl-featured here] (I recommend downloading this from within the add-ons option within Firefox for Android on your mobile device). I hope this helps, but if not, please come back here and we can look into a different solution for you.