The game isn't running smoothly. Animations are hitching sometimes.
Open the options menu (gear icon in the top right corner) and click the 'Optimizations' tab. Untick the 'Enable Threading' box and reload the page. If that doesn't help, there are more tricks in the full FAQ.
Which browser should I use?
Chrome (or Chromium Edge) is the best browser for Merc Zone. Firefox is also supported, but the game has been optimized to take advantage of features that only Chromium-based browsers support.