Thje highest...
macOS High Sierra - Technical Specifications
General Requirements
- OS X 10.8 or later
- 2GB of memory
- 14.3GB of available storage to perform upgrade*
- Some features require an Apple ID; terms apply.
- Some features require a compatible Internet service provider; fees may apply.
Mac Hardware Requirements
For details about your Mac model, click the Apple icon at the top left of your screen, choose About This Mac. These Mac models are compatible with macOS High Sierra:
- MacBook (Late 2009 or newer)
- MacBook Pro (Mid 2010 or newer)
- MacBook Air (Late 2010 or newer)
- Mac mini (Mid 2010 or newer)
- iMac (Late 2009 or newer)
- Mac Pro (Mid 2010 or newer)
macOS High Sierra - Technical Specifications
https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12
macappstores://apps.apple.com/app/macos-high-sierra/id1246284741?mt=12
Have a good backup & clone before installing, then these items may bbe of help in 10.13.6...
Mostly you just need a more modern Browser than Safari on High Sierra like Firefox or Brave.
https://brave.com/
https://www.mozilla.org/en-US/firefox/83.0/system-requirements/
Firefox 83.0 System Requirements (mozilla.org)
Opera…
https://www.opera.com/download/requirements
Opera system requirements | Opera
Thanks to dialabrain…
Firefox 112 is now the current version. runs in High Sierra.
https://www.mozilla.org/en-US/firefox/new/
Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…
https://www.libreoffice.org/
Thanks to Matti Haveri…
MS Office for Mac Mini - Apple Community
LibreOffice 7.0.4 works in macOS 10.11 and 10.14.
https://downloadarchive.documentfoundation.org/libreoffice/old/