Fabric

FO uses Fabric, instead of NeoForge, for the following reasons:

• Popular: there are at least 10000 Fabric mods on CurseForge and over 20000 Fabric mods on Modrinth

• Supported: most authors of NeoForge mods nowadays support Fabric as well, if not exclusively

• Compatible: Fabric mod developers work together to keep their mods compatible with each other

• Easy installation: there's a Fabric installer, which supports many launchers

• Fast updates: on the same day as Minecraft, and so do many mods

• Fast loading: roughly the same as vanilla Minecraft

• Small size: no huge libraries, just the Fabric Loader and the Fabric API

If you're coming from NeoForge, you can find a community-maintained list of Fabric mods corresponding to NeoForge mods.