This applies to the HTML5 client only.
1. Click the 
"hamburger button" to open the menu, then click 
Options.
2. In the options window, click 
Advanced, then click 
General.
3. Check the box next to 
Use hardware acceleration when available, then click 
OK.
4. Close all FireFox windows and wait for FireFox to exit completely, then restart FireFox.
On the other hand, if your FireFox is unstable and crashes a lot while playing the game, follow the same steps but 
uncheck the box instead.
 
