How to run autokey on startup in Ubuntu 20 (FF)?

Autokey likely has an option in its Preferences to have it autostart when you log in. In general, usefull for applications that do not present such option, you can set that up yourself.

In the Activities overview, search "start". You will see the icon for the "Startup Applications" tool. Launch it by hitting Enter when the icon is selected.

Add your custom application using the "Add" button. The "Add Startup Program" dialog appears. It contains three fields. "Name:" and "Comment:" are for your reference only. "Command: " should point to the executable of the program you wish to autostart. You can type the full pathname. If the executable is within the search path (i.e., you can start it at the terminal only by typing its name), then also here, only the name will be sufficient.

To know the name of the executable, you need to check the documentation of the package. For Autokey, fill out autokey-gtk in the "Command: " field.