I know a few people who updated their older (and eligible) Macs to Sierra and have had a lot of grief with their Macs slowing to a crawl and/or running hot. I want to avoid that fate with several Macs I own or look after. Their OS’s are getting long in the tooth, so I would like to know what is the best later OS for each one. #1 - Early 2009 iMac. 3.06 GHz Core 2 Duo. 8 GB RAM. 480 GB SSD (third party – Transcend), currently running Mavericks. #2 – 2012 Mac Book Air Maxxed out specs (whatever the best available chip was and 8 GB RAM), currently running Mountain Lion. #3 – 2010 Mac Book (white plastic) Core 2 Duo, 8 GB RAM, conventional (spinning) hard drive, currently running Mavericks. I don't really live in the Apple ecosystem, so the latest bells and whistles (and emojis) hold no interest for me. My main concern is having relatively recent Safari and Firefox browsers. Would appreciate your thoughts and experiences as to what is the best later Mac OS for each one. Thanks in advance.