Speaking about web services in general (not specifically Firefox Cloud Services), deleted files aren't deleted immediately. Since the files are usually replicated on multiple servers, it takes a little bit of time for all copies of a file to be deleted. Furthermore, it's usually more practical for a website to put files in a queue to be deleted in bulk on a schedule instead of individually.

I don't know specifically how Mozilla handles the back-end of image storage on their servers. However, when you delete a file, there's no way for you to recover it and it should eventually be removed from their servers.

However, this whole question will soon be obsolete since Mozilla is discontinuing the the Firefox Screenshots Cloud Services some time next month.

https://www.mozilla.org/en-US/firefox/67.0/releasenotes/

Users will no longer be able to upload and share screenshots through the Firefox Screenshots server. Users who want to keep existing screenshots need to export them before the server shuts down in the coming months.

https://blog.mozilla.org/futurereleases/2019/01/24/clarifying-the-future-of-firefox-screenshots/