Use Google as a Calculator
Google has enabled a calculator feature for users who utilize the Google search box in the Firefox browser.
From Google’s site:
To use Google’s built-in calculator function, simply enter the calculation you’d like done into the search box and hit the Enter key or click on the Google Search button. The calculator can solve math problems involving basic arithmetic, more complicated math, units of measure and conversions, and physical constants.
To read more about the calculator, as well as other Google search features, click here.