There are several reasons why you might be looking for a Firefox 91.0 ESR download today:

Mozilla Firefox 91.0 ESR (Extended Support Release) remains a significant version for users who prioritize stability and long-term compatibility over the rapid-fire updates of the standard browser. While newer versions have since been released, many legacy systems and enterprise environments still look for this specific build to maintain workflow continuity. Understanding Firefox 91.0 ESR

It is important to remember that Firefox 91.0 ESR is no longer the "current" ESR version. Mozilla eventually moved the ESR branch to version 102, then 115, and beyond. Using an outdated browser like 91.0 means you may be missing out on critical security patches released in the last few years.