This is my favourite calculator on Firefox (so much so that it's now my only calculator, aside from one in my phone for when I'm away from my laptop...) The only thing I miss in it is a unit converter (for that, I have to rely on another extension, which lacks a calculator, it only converts units...) Edit: I found a nasty bug! Entering a minus character in the beginning of a calculation crashes Firefox! After entering a minus character, the extension freezes and Firefox stops responding (tabs don't load any more, all extensions freeze...) Restarting the browser doesn't solve the problem, because the minus will be reloaded from the extension's history. The extension needs to be uninstalled to solve it...

More accurate than most calculators on the web. Handles complex math with ease. PEMDAS, no problem.

Best calculator after gnome-calculator

I think window is better than popup for calculator.
because popup are inconvenient to reopen if they lose focus when calculating multiple times.