If the corporate culture is corrupt and you do something corrupt that costs you your job, you can't complain that "everybody does it." Are you part of a Union? Is there a grievance procedure in place?
Your charge of falsifying Tax Certificates is a serious one. Do you have substantial proof, or is it all anecdotal evidence? I'll leave this to the employment lawyers in the crowd, but it sounds like you could obtain whistle blower status, though I'm not sure if that applies after you've been terminated.
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Texas has very limited grounds for wrongful termination. It does not appear likely under the facts you have provided that you have a meritorious claim. However, the facts you have not provided may be more important than the facts you have provided.
You can be fired for falsifying documents even if the company normally engages in that kind of behavior, although you may be able to prevail in obtaining unemployment benefits. You can notify the state about the fraudulent tax certificate allegations but because you were not fired for blowing the whistle on those allegations you do not have any protections in that regard.
However, you also mentioned that you have been with the company for 31 years. If in that time you have seen the company engage in widespread document falsification, why were you singled out for discharge? There are clearly facts you have left out, whether you are aware of the facts or not. Were other people discharged along with you for the same practices? Were you put on corrective action before you were fired? Is it possible your age had anything to do with your discharge?
Texas is a right to work state. Typically, unless an employee has an employment contract, or is employed under a collective bargaining agreement through a union, the employer can modify or terminate the employment at any time with or without cause. If an employer, at any time, decides they no longer want to employ someone, for any non-discriminatory reason, that employee can legally be terminated. However, an employer generally cannot terminate an employee for prohibited discriminatory reasons (such as racial discrimination), or in retaliation for certain protected actions (such as whistle-blowing). What you describe does not sound like prohibited discrimination or retaliation.
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Yes, I sympathize, but you did commit an improper act apparently in violation of company rules (even if rarely observed company rules). Terminating you for this offense is unfair. But, if you want fairness, you need to form a union or negotiate for fairness in an individual employment agreement.
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If a person under the age of 40 committed the same or similar conduct and was not fired you have potentially a prima facile case. They must be in the same or similar job as you.
Your replacement is under the age of 40 will support a viable case. if your replacement is over 40 your claim of age discrimination may be defeated.