Mac OS has not been updated from last 8 years

Back up your data, check the application compatibility table, click here, and then on the download link for a newer OS. A Mac running Sierra can be upgraded at least as far as High Sierra.

Catalina and above won't run 32-bit software and split the functions of iTunes up; if you choose one of them, use Go64 as well.

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How far you can upgrade macOS will depend on your MacBook Air's hardware model year.

If the hardware model year is

• 2010 or later – You can upgrade to High Sierra
• 2012 or later – You can upgrade to Catalina
• 2013 or later – You can upgrade to Big Sur
• 2015 or later – You can upgrade to Monterey

There is no possibility of upgrading past Monterey. If your MacBook Air was new enough to run anything newer, it would be too new to be running Sierra now.

Note that even Monterey is no longer one of the "most recent three".

If you can upgrade to Catalina or later, this would be good enough to let you run current versions of

• The Firefox web browser
• The LibreOffice office suite
• The GNU Image Manipulation Program (GIMP)
• The commercial Affinity V2 applications (Photo, Designer, Publisher)
• The backup utilities CCC v6 (not current, but still supported) and SuperDuper!

If you can't, FireFox 115 ESR will run on Sierra and High Sierra.

Mozilla Support – Firefox users on macOS 10.12, 10.13 and 10.14 moving to Extended Support Release

There are also old, unsupported versions of LibreOffice that will run on High Sierra, although you may have to dig around a bit in the archives to find out which ones are compatible with High Sierra.