I type the first letter. Firefox displays 10 options. I think they are bookmarks and recent google searches containing the letter. The ordering kinda corresponds to how recently or how often I used them, but it is not exactly that, for example the link I choose almost always somehow remains at the 2nd place. Whatever.
I type the second letter. Again, Firefox displays 10 options, bookmarks and recent google searches that contain the substring. At that moment, the link I want to click is quite often displayed somewhere. So I move my mouse...
...but a fraction of second later, Firefox loads "things it thinks I might want to search" (probably popular search results by other people that start with those two letters I typed) and inserts them into positions 2 - 5, moving the other search results below them...
...and just as I am pressing my mouse button, the link below my mouse cursor is replaced by google search for something completely irrelevant.
Aaaaargh!
(I wonder whether it is an accident, or on purpose, that the most frequently used link goes to the 2nd place, not the 1st one, so it gets replaced by an irrelevant google search. I suspect it probably increases some metric somewhere, and is probably interpreted as a good thing.)