Depends on what you got fired for.

23 y/o CFI flunked RJ training at his first regional: Probably not a big deal long-term if lessons are learned, and subsequent 121 training record is good. No trouble getting hired at another regional in this case, although there will be a delay getting called by majors.

43 y/o RJ CA shows up for work drunk, or gets in a fight with another employee: Long uphill battle. Only career option may be mesa.

Felony conviction: Game over for majors (possible rare exception for HIMS grads).