"Fire off" is an expression used to refer to the act of rapidly sending or launching something, often with a sense of urgency. There are several other synonyms for this phrase that can be used in varying contexts. To "shoot out" is often used when referring to sending a quick message or email. "Launch" or "set off" can be used when referring to launching an object or a missile. "Trigger" is another synonym that works well when describing initiating a process or setting off a reaction, while "blaze" can be used when describing a sudden burst of activity or energy. These synonyms for "fire off" can help add variety and nuance to your writing.