Invited by your recent email, I downloaded and installed Firefox v.100 onto my old but still very trusty MacBook Pro laptop, which can't reliably run anything later than El Capitaine. It downloaded and installed ok, but then refused to work, with a message that v.100 needs at least High Sierra to operate. There was no warning of this prior to installation. I searched the disc with Disk Drill, hoping to resurrect the older version of Firefox. However, it seems that v.100 has overwritten the previous version rather than deleted it, since Disk Drill could find no trace of any version, deleted or otherwise, other than v.100. I note that you retain backnumbers of Firefox on your website - so my question is "Which one should I download that will reliably work on El Capitaine?" I'm currently forced to use an old version of Safari as a browser on the laptop which I dislike intensely. I'm submitting this question from my desktop machine running High Sierra, on which v.100 works very well. Many thanks in advance for your assistance. Gareth Bennett