In Firebug, the command line of JavaScript console maintains its own history of executed expressions and so, you can easily go back and execute an expression again.
This is true only when the history is not too long because otherwise there is no way to get the entry you want.
For reproduce the problem just try to push the run button several times after inserting some code.
So the solution could be to claen the history of the JavaScript command line. Is it possible?
Or there are other ways to fix this problem.

I had this same issue. I disabled then enabled firebug, then closed and restarted Firefox. The history is now gone.
EDIT: disabling the add-on was not needed. Just restart the browser and the list clears.