Firefox adapts to Windows "High Contrast" themes differently than other browsers. You can switch to always using page styles, if you prefer, even with a high contrast theme. You would do that here:

"3-bar" menu button (or Tools menu) > Options > Content

Click the Colors button, then change the "Override the colors specified by the page with my selections above" drop-down to Never and OK out. Pages then should appear using the styles specified by the site.

Does that fix it, mostly? There might still be some remaining differences, but that should get you started.

Haggis said

After fitting a new SSD following a hard drive failure, and reloading Windows 7, I decided to switch from Chrome to Firefox. Now all my regular websites show subtle differences, as if some of the detail has been missed out. One forum I visit no longer has the grid separating each topic, author answers etc. When I went to ask this question here, I had to search for the box to type in the question by moving the mouse around till it turned into a cursor. Some formatting is wrong compared to Chrome, and some links are difficult to find. I've tried all the suggestions I can find in Help and Support.

Thanks, jscher, that's brilliant! That seems to have done the trick. As is often the case, the answer is simple, once you know it. Respect!

Thank you!! I had no idea what I'd done to my browser! I tried refreshing it multiple times, uninstalled and re-installed, all to no effect. Thank heavens I found your post.