I'd suggest a button for the upgrade menu. Right click isn't used too often in browser-based games, so if I hadn't expanded the description below I would have totally missed that it even had upgrades. I kept thinking it might be the kind of game where you have do mine so much first (or whatever) before the option comes up so I clicked and clicked, and was about ready to call it quits.
Font is kind of hard to read also as other have noted. Otherwise, fun so far! The graphics style is nice