Why doesn't "pgrep firefox" work on Ubuntu 20.04

As the title suggests, the following command on Ubuntu 20.04: pgrep firefox ... returns no results. Meanwhile, the following command: ps aux |grep firefox ... returns 6 results, 5 of which should match pgrep firefox. What's wrong with the original pgrep command on Ubuntu? ubuntu 81646 1.9 7.9 3063696 300208 ? Sl 13:12 0:29 /usr/lib/firefox/firefox -new-window ubuntu 81891 0.2 3.8 2473448 144796 ? Sl 13:12 0:03 /usr/lib/firefox/firefox -contentproc -childID 1 -isForBrowser -prefsLen 1 -prefMapSize 222536 -parentBuildID 20200720193547 -appdir /usr/lib/firefox/browser 81646 true tab ubuntu 82020 0.1 3.7 2438972 141396 ? Sl 13:12 0:02 /usr/lib/firefox/firefox -contentproc -childID 3 -isForBrowser -prefsLen 897 -prefMapSize 222536 -parentBuildID 20200720193547 -appdir /usr/lib/firefox/browser 81646 true tab ubuntu 82051 0.0 2.8 2401248 107520 ? Sl 13:12 0:01 /usr/lib/firefox/firefox -contentproc -childID 4 -isForBrowser -prefsLen 6935 -prefMapSize 222536 -parentBuildID 20200720193547 -appdir /usr/lib/firefox/browser 81646 true tab ubuntu 83658 0.0 2.0 2387704 78496 ? Sl 13:12 0:00 /usr/lib/firefox/firefox -contentproc -childID 6 -isForBrowser -prefsLen 10362 -prefMapSize 222536 -parentBuildID 20200720193547 -appdir /usr/lib/firefox/browser 81646 true tab ubuntu 90700 0.0 0.0 17664 664 pts/0 S+ 13:37 0:00 grep --color=auto firefox Running the same command on Ubuntu 18.04 and macOS works just fine.