When can you sue an employer for firing you without cause?

Absent a contract, employees in Texas are employed "at-will", which means either the employer or the employee can end the relationship with or without cause and with or without notice.  However, an employer cannot fire an employee for an illegal reason, such as age, race, gender, religion, disablity, color, national origin, for using or to prevent use of an employee benefit plan, such as pension or retirement or refusing to commit a crime.  That said, the anti-discrimination laws (all of the above except the last 2) only apply to employers with 15 or more employees. If you have a good faith belief that the real reason you were fired, regardless of the stated reason, is your age (over 40), then you can file a complaint with the EEOC. There is an office in most of the major cities in Texas.  If you have a good faith belief that the reason you were fired was for using an employee benefit plan, contact the US Department of Labor.  Again - offices in most major Texas cities.  If you are going to contact one of the federal agencies, do so immediately.  There are very short deadlines to do so.  At some point, you will have to provide evidence of any unlawful motive.  You can't just make a bald allegation and expect that something positive will happen.  Good luck.