This article describes what steps to take if your scroll position is not remembered when you go back or forward.

Caching

Certain websites tell Firefox that they should not be cached (saved temporarily). When sites do this, Firefox is unable to remember your scroll position.

In addition, there may be third-party Firefox extensions that can help you. Go to addons.mozilla.org to search for and install extensions for Firefox.

For more information about add-ons, read Customizing Firefox with add-ons.

Based on information from Scroll position on web pages not remembered (mozillaZine KB)