dstiles - 8:37 pm on Apr 2, 2013 (gmt 0)
Haven't yet found the answer to the top-panel problem BUT...
I rebooted today because Firefox's tab-mix-plus refused to show me values - none of the tick-boxes showed tick/no-tick - and I thought a full reboot would fix my problem with the metacity problem (last para above).
Well, it did sort-of solve the metacity problem (though not the firefox/tab-mix problem). When the screen returned after reboot it was a bright blue instead of my orange image background. And there were no menus or panels - just the desktop! In short, no way to navigate. Scary!
Tracked down a useful site from another machine (newly-installed Mint yesterday, was Ubuntu 10.04). Ctrl-Alt-Del and logout. On being presented with the login panel select the Sessions icon on it. This gives a menu of desktops. I selected Mate (my original) and got back everything just as it used to be - except for the top panel, of course. :(
Now to solve the tab-mix problem. :(