I am using a Raspberry Pi 4 Model B as my desktop computer with Raspbian Buster installed and everything works great except that some videos on Youtube don't have any audio. If I download the video and play it with Totem, then I can hear the audio. Is there a package that I need to install in order to hear the audio in my browser? I am using firefox-esr.
Try playing the downloaded file with Firefox using File->Open.
If doesn't work, check that you have the AAC codecs installed using ffmpeg -codecs.
If playing the local file does work, then, as outlined here you should start Firefox with MOZ_DISABLE_CONTENT_SANDBOX=1 env variable, or whitelist the directory where the codec libraries are located in, e.g. security.sandbox.content.read_path_whitelist=/usr/local/lib/.
Also note that on Raspbian Chromium seems to have better support than Firefox. Apparently, the Firefox ESR build currently available in Raspbian supports none of the settings that are discussed above. So the only option is to use an alternative build of Firefox or a different browser.
I have just checked your test video and it works fine in the latest Pale Moon, with sound.
Pale Moon browser is a fork of Firefox which supports older Firefox addons (which Firefox dropped after version 52). It is slower than Chromium but requires significantly less RAM, so it can actually outperform Chromium on a system which is short on memory.
Pale Moon can be installed by following instruction here. For the latest Raspbian, the installation instructions are:
echo 'deb http://download.opensuse.org/repositories/home:/stevenpusser/Raspbian_10/ /' > /etc/apt/sources.list.d/home:stevenpusser.list
wget -nv https://download.opensuse.org/repositories/home:stevenpusser/Raspbian_10/Release.key -O Release.key
apt-key add - < Release.key
apt-get update
apt-get install palemoon