Hello,

in web browser Firefox, I want to tick one of many checkboxes on two websites. Each checkbox has different description.
Also I would prefer if there is a Firefox extension that prefill certain mine selected form field on two websites.

Is there any such extension?

I have only found this:
https://addons.mozilla.org/en-US/firefox/addon/checkthemall/ - allows you to select many checkboxes and then check / uncheck / invert them with one single click
https://addons.mozilla.org/en-US/firefox/addon/unchecker/ - check or uncheck all your checkboxes at the same time - low permission needed
https://addons.mozilla.org/en-US/firefox/addon/drag-select-checkbox/ - Checks all checkboxes within a click + dragged area, deselects if all are already checked. Save time by dragging over a number of boxes instead of clicking each one.
https://addons.mozilla.org/en-US/firefox/addon/checkbox-selector - Press the Shift-Key and make a rectangle-selection with the mouse by clicking-and-dragging. All checkboxes within the selection will change their status (unselected will be checked - and vice versa)