Since official Mozilla mirrors typically push users toward the latest version, you will likely need to use a trusted archive like PortableApps.com.
I can help you find specific extensions that work with version 81, or I can show you how to transfer your current profile into the portable folder. Just let me know what you need! firefox 81 portable download
A portable app is a software package that runs without an installer. This offers three major advantages: Since official Mozilla mirrors typically push users toward
Keep it on a USB drive and use your exact setup on a library computer, work laptop, or a friend’s PC. or a friend’s PC.