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.

If you’ve deleted applications/programs, and they have leftovers in the Library/Application Support/ folder, then it is probably safe to clean out that program’s Library/Application Support/ 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.