They have quite distinct characteristics. Firefist is often shown as powerful and aggressive, while Deadpool is known for his humor and unpredictable actions.
I don't think Firefist was ever part of Marvel Comics. Marvel has so many characters and storylines, but Firefist doesn't fit into any of them that I'm aware of.
Since it's a fanfiction, it's likely a creative take on the Harry Potter world. The 'firefist' might be a magical power that is not in the original series. The 'ace' could be a code name or a title for a character. It could be a story that delves into the backstory of how this power was discovered or created. It might also involve a quest to master this power, with the character facing various challenges and obstacles along the way. There could be new magical creatures or spells introduced in relation to this 'firefist' ability as well.
You can try looking on popular fanfiction websites such as FanFiction.net or Archive of Our Own (AO3). These platforms host a large number of fan - created stories, so there's a chance you'll find the 'harry potter firefist ace fanfiction' there.
Deadpool became Deadpool through a series of complex and sometimes convoluted events in the comics. He was originally a mercenary named Wade Wilson, who underwent experimental treatments that gave him his unique powers and personality.
There is a loose link. The version of Deadpool in Wolverine Origins provided a starting point, but the standalone Deadpool movies developed his character and story in a more distinct and expanded way, deviating from the initial portrayal to create a more focused and engaging narrative.