If any of these "pre-Big Sur" computers run Catalina, you could install Firefox. The latest version of Firefox still runs on Catalina.
If they are running Sierra, High Sierra, or Mojave, you could install Firefox 115 ESR. This is an older version of Firefox that was scheduled to stop getting security updates from Mozilla long, long ago. But Mozilla has been giving it lots of "stays of execution" and is now saying they will provide security updates for it until March 2027.
Mozilla Support – Firefox users on macOS 10.12, 10.13 and 10.14 moving to Extended Support Release
If they are running something earlier than Sierra, you are probably out of luck. For example, I don't know if you can still download Firefox 78 ESR (which runs on El Capitan, among others), but even if you could, it is ancient, and has not gotten even "critical security updates" for several years.