As for keyboard shortcuts, userscripts can do that by observing individual key presses. I personally use a tiny library [0] by the Violentmonkey team to do it which is easy to use and smooths out differences between platforms and browser implementations.
While userscripts can't add items to toolbar or context menus, you can register menu commands [1] which have the added benefit of being able to be added dynamically after examining page contents. This is done using the `GM_registerMenuCommand` api [2].
[0]: https://science.nasa.gov/universe/black-hole-week/