In Firefox 77 and newer, Mozilla has introduced an "enhanced" URL bar, called "Mega Bar". As soon as you click on it or it gets the focus, it pops out and gets bigger. It also creates a drop-down list with your last searches, the sites you last visited and search suggestions.
To many people, this is a big mess and they would like to disable it. Here is how to do it:
Type about:config in the URL bar. Accept the warning message.
Paste the following string into the config search:
Click on the + button to the right of it. This will create this config with a setting of "True". From now on, you will get the classic URL bar.