It's really nice! The only thing is that I'm very lazy placing parentheses (especially for long equations). So 3+9)/4 for example instead of (3+9)/4 it would be nice if that would also work!

There's probably something I don't see, but I can't find how to actually use this extension. I'm typing simple math in the address bar (ex: "2+2") and I only see google results.
What am I missing?

Nice, but could you add percentage? For example, 120+10%=132.
And how to access history in a new tab without adding a new entry to it?

Excellent one. Don't wait for crappy server connection if you don't have enough bandwidth. Keep up good work.

Simple and powerful, doesn't rely on external servers :)