You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• Hold down the Shift key and left-click the Reload button
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Command + Shift + R" (Mac)

Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Clear the cache":

• Firefox/Options/Preferences -> Advanced -> Network -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Firefox/Options/Preferences -> Privacy -> "Use custom settings for history" -> Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• Do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

I had no problem with the map It zoomed in and out when I used the mouse scroll wheel or the plus or minus icons on the map.

https://www6.hertsmere.gov.uk/online-applications/applicationDetails.do?activeTab=map&keyVal=OG410YIFMUV00

I know that the map responds to the mouse in that way, but this is not the behaviour I expect. The map should automatically zoom to the planning site where the development is proposed. Try it on IE or Edge...

Thanks for your reply, Fred, but the outcome of using IE or Edge is the screenshot attached. See the property outlined in red.

Firefox simply shows a large scale map, without locating and highlighting with a red outline the selected site plan.

Nobody seems to know why there's this significant difference in display between the browsers....