Sometimes the iceweasel browser does not produce a horizontal scrollbar
even though the page being viewed is wider than the browser window. At other
times the scrollbar will appear when viewing the same website.

Point the browser at http://www.national-lottery.co.uk/

Sometimes a scrollbar appears at the bottom. Other times it does not.
Toggling the stylesheet on and off as follows may cause the missing scrollbar
to magically appear:

Menu, View, Page Style, No Style, Menu, View, Page Style, Basic Style

Can you upgrade to version 14? 12 is outdated.

Mike

src:iceweasel has been superseded by src:firefox-esr in version
45.0esr-1 in March 2016. Transitional packages to ease upgrades were
provided in the wheezy, jessie, stretch and buster releases. The
transitional packages have been removed finally before the bullseye
release in August 2021.
After regular security support for buster ended in August 2022 and LTS
support ended in June 2024, I'm closing the remaining bug reports now.

Andreas