Hmm, I don't see it on Windows 7, either (just Arial). I think that might be because Firefox views Arial Black as an extra-bold variant of Arial rather than a distinct font family of its own.

If you still have access to an older installation of Firefox that you previously set up to use Arial Black, I could suggest how to research the particular settings stored in that Firefox to see whether there is a way to copy them over.

Otherwise, an add-on or custom style rule might be able to modify the pages.

Thanks very much for your reply jscher2000. Didn't expect a reply so soon!

Both laptops (one running Windows 7 and the other Windows 10) are running the latest version of Firefox (currently 54.0.1 32-bit). The Firefox font customiser on the win7 laptop shows many fonts like Arial Black not shown on the Win10 laptop (and some vice versa). I'm trying to find a suitable dark font to use on the Win10 but alternatives like "Elephant" wouldn't be too easy on the eyes! I might see if can find and add in a new font somehow.

Not sure how to go down the "add-on or custom style rule" route? I'n not really a web expert thought it would be great if there were a registry entry or script file I could edit to fix the problem.

. If I'd known about all the hassle in moving to Windows 10 for people with poor eyesight (microsoft is no help either) I might have stayed with Windows 7).

Thanks again for your help

On Windows Firefox may not distinguish between the various fonts that belong to specific font-family and treat them as one font family. A specific variant is chosen based upon what font style and variant and weight rules are specified via the CSS properties.

E.g.: Arial Black would be: font-weight: 900; Arial Narrow would be: font-stretch: condensed;