If you’ve deleted applications/programs, and they have leftovers in the Library folder, then it is _probably_ safe to clean out that program’s Library subfolder. If, for example, I wanted to remove Firefox’s old data after uninstalling, I would go to Finder’s search box and type `firefox`. Make sure the search is set to `This Mac`, then click the plus button underneath the search box to add `System Files` and `are included`. You’ll get a mix of small files and directories as a result (ideally). The folders are what you want to focus on. Clicking on a folder should show its path at the bottom of the finder window. If a Firefox folder is in USER/Library/Application Support/ I just delete the folder. Deleting the folder will also delete its content, so you may see your total number of search results drop as a consequence.

That being said, be careful. If in doubt, don’t delete it without referring to that programs documentation.