Yesterday, I accidentally pressed Ctrl and another key which I do not remember anymore. This triggered my Tor Browser to look like this:
How can I get rid of this grey bar at the bottom?
This looks like Tor Browser's letterboxing feature: https://support.torproject.org/tbb/maximized-torbrowser-window/
This probably happened when you resized the window. If you make the window slightly taller, the webpage should fill in the bottom section.
This seems like the letterboxing feature which is used to avoid fingerprinting.
Try going to
about:config, and turn
privacy.resistFingerprinting.letterboxing to false to get rid of it.