First, check this setting; https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

If Firefox loads an older version from the disk cache then you need to clear the disk cache when you close Firefox.

You use "Clear history when Firefox closes" to clear the Cache

• Settings -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings
• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exceptions for passwords and other website specific data
• clearing "Cookies" will remove all cookies including cookies with an allow exception you may want to keep

Have you checked that the url is still correct?

Thank you all for your help. The only thing that I have tried was to disable all of my add ons, which cured the problem. I don't know why it worked but have since re enabled the add ons and everything is still OK, so I can say this case can be closed !

That was very good work. Well Done. Please flag your last post as Solved Problem as this can help others with similar problems. Go to that post and click the 'Solved' button to its right.

Solved problem.