MARCA

You can't generate a logo in adobe firefly, but you can generate a vector illuxtration which later can be a icon for a logo https://www.adobe.com/products/illustrator/text-to-vector-graphic.html
and you can generate a text effects for lettertype logos here
https://new.express.adobe.com/new?category=text&prompt=floral+hand+embroidery+pattern&primary-seed=76291&seeds=&action=text+effects&width=1920&height=1080&text=Thread&fit=tight&style=hyper+realistic&fontPostscriptName=CooperBlackStd

You've asked for a brand containing letters; in general, AI tools don't do a good job in generating legible text. 

You have posted in a community forum on using Adobe Firefly. Actual image generation happens on the Firefly website: https://firefly.adobe.com

If you have a question about using Firefly, feel free to post here!  If you are new to Firefly, here are two videos to get you started Adobe Firefly (Shawn Kosel) and Adobe Firefly Basics | Office Hours . Also, here are some tips for using Firefly: Writing effective prompts.

If you want to create a logo, you might find that using Adobe Illustrator to create a vector graphic might be the best Adobe tool For more information on using the generative AI features in Illustrator, please see: Generate a vector graphic using a text prompt