I have a cheap Lenovo Chromebook Flex 3 with a MediaTek MT8183 processor that I picked up for under $79 at a BestBuy Black Friday sale. I run Firefox ESR on it with no problems in the Linux container, no flatpak needed. Here's all you need:
I have a Lenovo Chromebook Duet (the first gen one) which I think runs the same processor. Though the 4GB RAM is a little bit limiting so I don't use it that often.
sudo add-apt-repository ppa:mozillateam/ppa
sudo apt-update
sudo apt install firefox-esr