Maybe try new drivers?

Thanks for the reply, Finitarry, but I'm afraid I'm not very technical so unfortunately I am really not sure what you mean by "drivers" in this context, let alone how to act on that. The only thing that occurs to me is that you might mean that I should replace the mouse with a new one. If so, then I think it unlikely that that would work as the problem doesn't seem to lie with the mouse, rather with Firefox itself. After all, every other program I run responds just fine to the scroll wheel, and that includes not only common programs like Microsoft Word etc but also other browsers such as Safari and Google Chrome. Or maybe I simply haven't understood your reply?

Hi, not sure if it will be that, but to check for a driver update, type Driver into the search box on the Start Menu and select 'Update device drivers,' then open 'Mice and other pointing devices' > Driver tab.

I would suggest you try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.