Uploading Reference images to train for creating new Gen AI Images

Firefly does not have this capability (like what NightCafe has). You can, however, use a single reference image to influence the look and feel of an image you are creating using what Firefly calls a Reference Image.  You copy that image into the reference image section on the right panel in text to image:

This feature is discussed in more detail in this help article:

Generate images to match the style from an existing image 

and demonstrated in this video:
How to use a Style reference image in Adobe Firefly