Hi,
Starting in Firefox version 157, you can enable Compact mode in Firefox, by going to the Window density section in Firefox settings for Appearance.
In earlier versions of Firefox, there's a workaround for advanced users to activate Compact mode, which involves the Configuration Editor (about:config page). However, for the best experience, we encourage you to use the default Density setting in Firefox version 156 and below.
For more information, see the Enable Compact mode in Firefox article.