Some webpages (like office 365) override the contextual/right-click option, so Firefox's "Take Screenshot" option will not appear. I know that I can add the screenshot button to the toolbar, but I only need it for the rare case where the contextual icon is disabled. To me, it makes sense to include this in one of the menu options somewhere. Most software that provides a contextual menu option also provides a way of accessing that option from the main menu (especially for functionality where you don't actually need to click on an individual element for it to work... like screenshotting a webpage).
I don't think this necessarily needs to go in Tools > Browser Tools > Take Screenshot, but that seems like a logical place for it.