Calc

This Calc add-on results really useful when you need to do some maths operations while surfing the web.

It's not the first time you have to access the Windows calculator from the Start menu and that takes some time and clicks... from now on and thanks to this ad-on you can have a calculator in the sidebar of your Firefox.

It will let you do basic calculations like sums, rests, multiplications or divisions, but it also offers you the possibility to perform percentages, factorials, logarithms and more

Though Calc is very useful, its interface is not very eye-catching.