When I first got to see the new site I had to post several answers before I could work out what as it was only on my screen so fleetingly. (I only see it for a fraction of a second). So there's definitely something odd going on if you're seeing it for far longer than I do.
Check that there's nothing hogging nearly all of your laptop's resources:
Right-click on the taskbar at the foot of your screen, select 'Start Task Manager' and click on the performance tab. 'CPU Usage' shouldn't be massive. (Mine is currently moving between about 30% to 40%). The 'Memory' display shouldn't be too big either. (The blue line on mine is about half way up the graph). If either of those figures looks abnormal, let us know.
If you don't find anything unusual in Task Manager, try running Firefox with your add-ons disabled:
Ensure that you've got the Menu Bar (File, Edit, View, etc) visible at the top of your screen. (If it's not there, right-click near to the top and put a tick next to 'Menu Bar'). Then click on Help, followed by 'Restart with Add-ons Disabled'. If the problem disappears you'll know that it's an add-on which is causing your problem. (Restart Firefox as normal, and then disable your add-ons one at a time to find the culprit)
If you still haven't solved the problem reset Firefox:
Click on Tools > Add-ons > Extensions.
Write down the names of any extensions you've got (and wish to keep) as you're about to remove them.
Go to Help > Troubleshooting Information and click on Refresh Firefox.
Return to Tools > Add-ons > Extensions and then search for, and reinstate, the extensions you require.