I'm having trouble envisioning this. Do you mean when you scroll, sometimes a line is repeated, or is it bigger than that?

The page loads like this initially but when you refresh it its fine again.

Did you make any changes to network.http prefs on the about:config page or do you have extensions that make such changes?

Try "Reset all user preferences to Firefox defaults" on the Safe mode start window - See http://kb.mozillazine.org/Resetting_preferences

Hi thanks for you help with this.

I have tried running this in FF safe mode but the problem still persists.

The way to recreate the issue is browse to http://www.fco.gov.uk/en/ and if you repeateadly click on the FCO crest on the Large blue banner few times you should be able to replicate the issue.

Any help with this is greatly appreciated.

For mysterious reasons, Firefox occasionally doesn't "complete" laying out a page. Whether this is due to something funky in your cache, timing of files arriving from the server, a false "reset" or "stop," or who knows what, there is not always a satisfactory explanation. If it were a site I used a lot, I would dig into the behavior in more detail, but...

One thing you might try is to occasionally use Ctrl+Shift+r when you reload. This should bypass any older cached files for the page that may have changed and that may be causing inconsistent layout instructions.