Here is a small annoyance with an easy solution. The newer versions of Firefox and Iceweasel have added an "inspect element" tool to the right click context menu. It certainly look very pretty and will be of use for some folks. But its confusing for people like me who use Firebug. You see Firebug adds an inspect element to the context menu too.
So, to disable that other inspect element option head on over to about:config, promise to be careful if you need to and change devtools.inspector.enabled to false (double clicking works). Job done. I told you it was easy.