What you specify in your application with regard to your work experience is at your discretion. You may be asked about it in interview, and it's not a good practice to lie. Still, there will be no background check into your previous employment. The major factor is your scholastic performance, MCAT, LORs and to some degree, how you present at interview.
If your story is not a negative one, ie "My boss accused me of stealing," or "My boss always hated me," then you may be able to honestly tell the story during interview without trouble.
If you are really concerned you MIGHT be fired, you should find a new job ASAP and "move on."
Remember that your applications and interview is your chance to put yourself in the best light possible without being dishonest. It is not a lie if you choose not to include why you left a job any more than it is to omit speeding tickets or a misdemeanor drunk in public violation from freshman year.
Don't worry yourself too much about it. Unless someone on the admissions committee knows your (former) boss, there will be no deep looks into employment. BUT DON'T LIE, YOU WILL GET CAUGHT.
OtoNerd B)