If you find the 'extra' suggestion feature in Firefox 43.0.1, when searching from the address bar, an annoyance:
... even if you have this unchecked:
... then you can turn the feature off in about:config
In a new tab type about:config ► Enter,
"promise to be careful"
then navigate to the following entry and double click on it to toggle it to false.
Firefox 44: (2016-01-26) Release and Beta versions of Firefox will not allow unsigned extensions to be installed, with no override.