It seems I'm yet to find the best grayscale solution for Firefox. Cons with this extension: it flashes color version for a moment on page refresh; pinned toolbar button requires extra clicks to toggle the extension. Nice that hotkey switch option is available, but not as comfortable as one click toolbar toggle would have been.

love that you can customize the intensity of the filter, just what I was looking for and couldn't find the functionality in any other addon

This is the best grayscale Firefox addon I've seen so far.

Wonderful addon