Google Chrome is the preferred browser. We also offer a Chrome plugin called Mastermind Search Companion, which can be installed from the Chrome Web Store:
https://chrome.google.com/webstore/detail/mastermind-search-compani/afjaifocdahgfpfgepaniahacjjoeeli
If you do not have Google Chrome installed, you can download it by following the instructions here: https://www.google.com/chrome/.
Mastermind is also accessible via Firefox, Safari, Internet Explorer, and Microsoft Edge - though some features may be limited.