I have an older iMac desktop (system is 10.11.6). It can no longer be upgrade to newer systems. I had firefox on it, but am doing some updating of security info and saw that there was a newer version of Firefox than what I was using. I tried to upload it but it said I needed to discard the older version. So I did that. Then when I tried to upload the newer version, it said I couldn't because my system is not compatible (needs at least 10.12).

How can I put the older version back on my computer?