Convert Mixamo rig bone names (as imported to Blender via FBX) to standard Blender bone names. This is especially use if your mesh uses the 'mirror' modifier. 1 - Backup your Blend. 2 - In action editor disconnect any animation connected to the rig. 3 - Paste the script in a text window and select "run script". 4 - Notice that all bone names are…

mixamoToBlenderBoneNames.py

# IMPORTANT: make sure no animation is assigned
# to the rig before you run this script,
# otherwise linked animation data will be corrupted.

import bpy

# ----------------------------------
# Mixamo left/right bone names start with 'Left'/'Right'
# Instead we apply Blender's standard .L/.R suffix
# and get rid of long suffix
# ----------------------------------

def makeStandardBoneName(x):
    x = x[10:]
    if 'Left' in x:
        x = x[4:]+".L"
    if 'Right' in x:
        x = x[5:]+".R"
    return x

# ----------------------------------
# Standardize bone names.
# ----------------------------------

for armature in bpy.data.armatures:
    for bone in armature.bones:
        n = bone.name
        if 'mixamorig:' in n:
            bone.name = makeStandardBoneName(n)

# ----------------------------------
# Convert animation channel names
# ----------------------------------

for action in bpy.data.actions:
    for curve in action.fcurves:
        p = curve.data_path
        if 'mixamorig:' in p:
            e = p.split("\"")
            x = makeStandardBoneName(e[1])
            curve.data_path = e[0]+"\""+x+"\""+e[2]

Life saver. Thank you sir