OK, Thanks.
Tried that - only have 4 extensions plus Firebug.
Installed them one by one in a new profile and Firebug displayed correctly before the others were installed and correctly after installing all the others.
Then went back to the original profile. removed them all, reinstalled Firebug and it worked correctly then reinstalled the first of the others - DoNotTrackMe. Firebug went back to the bad display. Removed it again and FireBug remained bad. So it looks as though that extension cannot get on with Firebug and even when it is removed it leaves Firebug in a bad way.
Any further ideas - obviously I can work with the alternative profile but it makes for an extra step when I want to work as a developer rather than as just a browser?
Richard