CurseForge now officially supports Fabric - Jumploader is no longer necessary!

This mod will stop Forge mods from loading - it does not allow Forge and Fabric mods to be used together!

Make sure you back up before you join worlds where you have used Forge mods previously - as all the blocks/items added by those mods will disappear.

Jumploader

Jumploader is a mod that allows the use of Fabric mods in CurseForge modpacks, by loading Fabric as if it were a Forge mod. It works on both the client and server, however it's only necessary on the server if you want to launch the game using tools that only support Forge. Jumploader is fully configurable to load any Minecraft version and mod loader, but by default it will load the Fabric loader and library versions corresponding to your current version of the game. It only loads if your game was launched from Forge/ModLauncher, so it won't do anything if your Minecraft launcher already supports Fabric.

What it does

Loads Fabric (or a different configured mod loader) instead of Forge, from a Forge installation.
Automatically loads the latest Fabric loader version for your version of the game, with no required configuration.
If necessary, loads the unpatched Minecraft game, so that Fabric can be used with it.
Ensures that Fabric and Mixin don't see the classes loaded by ModLauncher.

What it doesn't do

Jumploader doesn't let you run Fabric mods with Forge or Forge mods with Fabric - it hijacks the launching process so that Forge never loads. See Patchwork for a project aiming to let you use Forge mods with Fabric.
Jumploader doesn't currently work very well on the client for loading a Minecraft game with a different major version (including snapshots) to the currently loaded game, as supplementary assets (such as sounds) are downloaded separately by the game launcher. The Asset Validator mod fixes this issue.

Requirements

Requires a Forge version that uses ModLauncher (or ModLauncher on it's own, without Forge) - this usually means 1.13 or newer.

Support

Please make an issue on the Github issue tracker if you have any issues with the mod.

Updating Fabric loader

Jumploader installs the latest version of Fabric loader when you first launch the game, but it doesn't automatically update it afterwards to prevent breaking existing modpacks. If you want to change the version it installs, edit the pinFabricLoaderVersion property in the jumploader.json config file to the version you want.