This Calc add-on results are really useful when you do some math work while surfing the web.
This is not the first time that you can not access the Windows Calculator from the Start menu and from now on some time and clicking ... Thanks to this add-on and your Firefox sidebar can have a calculator.
It gives you basic calculations such as numeric, rest, quality, or sections, but it gives you the possibility to perform percentage, factorial, logarithm and more
Although Calc is very useful, its interface is not very interesting.