Browserhax is notoriously unstable on anything between 9.0-10.1. Members on GBATemp have tried dozens of times before getting it to work. I remember one person who alleges to have tried literally one hundred times before getting it to work. While you continue your attempts, remember to clear your browser data and vary how you move the slider.
When you inevitably get it working, you should install menuhax ASAP so you don't have to use browserhax anymore, so make sure you have that ready. If you have the homebrew starter pack, the menuhax installer should be included.