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. For more information on searching for and installing extensions, see Find and install add-ons to add features to Firefox.

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