Good news: It's possible to install Firefox in the ChromeOS Linux development environment using the regular Debian apt commands now, not just with Flatpak.
Bad news: It's a straightforward yet daunting task straight out of a '90s Linux hacking guide. https://support.mozilla.org/en-US/kb/run-firefox-chromeos
Other news: No matter which way it's installed, Firefox 148 on ChromeOS 144 is so unstable that it crashes unpredictably within a few seconds to a few minutes.
Mozilla's instructions for installing #Firefox on ChromeOS don't mention Firefox ESR.
I forgot that this kind of ESR was an option.
`sudo apt-get install firefox-esr` works just as well as `sudo apt-get install firefox` does. Today, it installed Firefox ESR 140.8, and it seems as stable as 148 should've been.