@zen: That’s strange. At the moment I can’t get errors when using BetterSearch and horizontal add-on. But that doesn’t say much.
So could you re-install BetterSearch, restart Anki and try again.
If it still crashes your browser please describe the exact steps to reproduce this problem. And also share the debug info that you can get like this: in the main window click on the “Help” menu, then select “About”. In the dialog that opens there’s a button “Copy Debug Info” at the bottom. Click it to get the text into your clipboard. Then share it. Hopefully this allows me to repriduce the problem so that I can fix it. This might help many other people.