Ideally, we would have a free labor market in which employers could hire or fire whoever they wished, so long as they did not infringe upon the terms of an employment contract. Employers would also be free to make contracts with employees or employee bargaining units. If labor unions limited their activities to peaceful bargaining, I would support the right of employers to enter into union security agreements.
However, labor unions often employ coercive tactics (organizing sit-ins on an employer's property, preventing shipments from entering or exiting plants, violently assaulting "scabs," etc.) The use of such tactics during labor strikes has given rise to labor laws which require employers to "bargain fairly" with striking workers. In effect, union violence and bullying has terrorized legislators into passing laws that give them an upper hand in the bargaining process, which is especially unfair towards non-union workers.
Until such time as all "fair bargaining" laws have been repealed and coercive actions by labor unions are actually punished, I support right-to-work laws. They are not ideal, but they are certainly better than the current situation in non-RTW States, which is unjust towards non-union workers.
And no, I don't support right-to-fire laws
That's a strange term to use, considering that right-to-work laws prevent employers from firing employees for non-membership in a labor organization.