Thanks for spotting this! It is a known bug and has been fixed and rushed into beta testing (in Mozilla speak, this is called uplift). The fix is currently in Firefox beta and is scheduled to be released at the end of the month.
There's a bug logged here: https://bugzilla.mozilla.org/show_bug.cgi?id=1124884
And a support article on how you can clear your search history here: https://support.mozilla.org/en-US/kb/clear-your-search-histo...
Hope this clears things up a little!
Jayelbe