ContextCalc is a lightweight extension which adds a menu item in your Firefox context menu, i.e. the menu which you can popup right-clicking on a webpage.
It shows the result of the computation of a user-defined formula/equation using the numbers found in the selected portion of the webpage when the context menu was invoked.
This extension is very useful to compute repetitive formulas when doing web researches. E.g. many websites show items with prices which are not including the VAT tax; entering the formula "x*1.21" in ContextCalc allows you to immediately know the prices VAT-included (1.21 is for VAT equal to 21%): just select the price on the webpage and right-click!
The extension supports lots of mathematical functions and the most common constants.