Is it easy to be denied top secret security clearance?

Depends on why you were fired, what the job was, and who did the firing. If you did not commit a criminal act leading to your termination, then it’s probably no big deal.

No one can tell you whether you will receive a Top Secret Clearance. The reason you were fired will be a factor. Be completely honest on all forms filled out or questions asked.

Just make sure that you are completely honest about everything when you fill out all the paperwork and when they interview you and ask any questions. If they find out you are lying or hiding anything then you will be denied for sure.