This add-on for Firefox is used for finding zero-width characters on pages, it shows how many characters, how many bytes they take up, and the ability to copy them into the clipboard.
On the preferences page for the plugin you can whitelist sites for the add-on to ignore, and an option to choose what you want to replace with, the default being 🕵 (if you want to strip characters, just set it to nothing).
Get the add-on here: https://addons.mozilla.org/en-US/firefox/addon/zerowidth-detection/