https://support.mozilla.org/eu/questions/1022724

This suggests you can go to about:config and set devtools.toolbox.host to bottom for the toolbox.

However, it also suggests that the Parent process toolbox you are looking at can't be docked.

It's possible the one you saw before was the Web Console (Ctrl+Shift+k) which is docked by default. On that one, there are a couple of icons on the right side to re-dock either on the bottom or side of the tab content. I don't know whether it is possible to dock the Browser Console.

Try using Ctrl+Shift+K to open the console for a specific window/tab (this one should be dockable) instead of Ctrl+Shift+J, which opens a console for every window/tab combined (so can't be docked to a specific window/tab)

(Also who downvoted me for being the only one to even attempt to offer a solution, geez. Makes people not even want to try to help.)