The "worse offender" argument is a tough sell to a jury who will judge you on your use of marijuana. That's akin to saying, "Well, they let other people sleep on the job but when I was caught. . ." Although you may have a theory, the practicality of it being successful is very slim.
Florida is a state where employment is "at will." That means that a private employer can make up their won rules about their employees and can fire someone for the slimmest of reasons. If taking a drug test was not in the employee handbook maybe you should have refused to take it. If you were denied a claim because of something invading your privacy and not noticed in the handbook you might have had a better claim.
Now that there is evidence of illegal drug use, they have a reason to fire you, whether you agree with it or not and even whether they need one or not, which they don't.
As to what other people have done and/or gotten away with, the previous answer is correct - that is a tough case to sell to a jury. A civil case costs a fair amount of money to file these days and the defendant will surely fire a motion for summary judgment which says that since there is no material facts in dispute please rule on this case according to the law.
There really are no disputed facts: you took the drug test. It was positive. That's a violation of their policies. They fired you as they are able to do. In such a scenario the judge would have little choice but to dismiss your case then and there. The case would likely never reach a jury so the argument of selective enforcement, which is a tough sell as noted, would never be heard.
Actionable discrimination claims in an "at will" state usually revolve around age, gender, religious, free speech or other areas protected by the United States Constitution and not unique local policy decisions of the company.
I'm sorry this happened to you but I do not think legal action is going to be an appropriate avenue to pursue.
No, unless you were singled out on the basis of being a member of a protected class (race, religion, gender, ethnicity, handicap, etc.). Otherwise, an employer is Florida may fire you for ANY reason or no reason at all, with or without notice.