There can be a number of reasons this might happen, most of which are out of your control. These reasons include: The website's network has a problem and can't send you the style sheets. The website's code isn't using their stylesheets properly (perhaps they recently deployed some new code that doesn't work properly).

Thank you for your reply, however this issue is not occuring with other web browsers on the same PC. The issue is only with FireFox.

It is also not an issue on my Android devices.

Without a URL for anyone to try no one can tell what is happening. And if this is a login site that makes diagnosing the issue more harder since until someone can login.

The two website addresses are; https://www.diy.com and https://www.trade-point.co.uk

The images that I attached to the original message have both the addresses.