Simple. Fast. Re-target.

Usage:

1. Import and scale Mixamo rig to match Rigify
2. Select Mixamo source and Rigify target in UI (in Item tab, above Rigify panels)
3. Press Retarget button. Your character now follows the Mixamo animation
4. Press Bake. Rigify now has the animation baked to its FK controls.

Tips:

• Upload your model to Mixamo when exporting animations. This gives bone placement and proportions more closely matching your character making re-targeting more accurate. Also allows you to easily see when the Mixamo rig is scaled to the right size.
• Want IK? Use Rigify IK->FK snapping tools in key poses. I do not recommend using the IK->FK for all keys function, it takes a long time and gives bad/jittery results when IK is keyed on every frame. Just pick out the key poses for IK.
• Don't want root motion? Just delete all the keys produced for the Rigify root and your animation should look the same just without Y axis displacement.
• Something broken? Only works in Blender 5. Assumes you didn't rename any of the default FK controller names in Rigify or Mixamo.
• I also added a Rigify quick mode switch drop down to put all limbs in IK, FK or User defined modes.