Shell scripting, check for a folder, copy to it if it exists then self destruct

Thanks to my heavy workload at the moment I'm drawing a blank on piecing this together. I can piece together bits and pieces of this whole task but for the life of me I can't put it all together.

I'm wanting to build a script that will execute when a Mac user logs in. The script checks for the presence of a folder ("~/Library/Application Support/Firefox" specifically). If it finds the folder it copies a file into one of it's subfolders and then removes itself, along with the login hook needed to run the script in the first place. If it does NOT find the specified folder then it exits quietly and runs again the next time the user logs in.

Any ideas?