Mac storage basics · Caches, logs and temporary files
How do I clear Firefox's cache on a Mac?
Short answer: In Firefox Settings → Privacy & Security → Cookies and Site Data, click Clear Data and tick only Cached Web Content to keep sign-ins.
In Firefox, open Settings → Privacy & Security, find Cookies and Site Data, click Clear Data…, tick only Cached Web Content and confirm. Leaving cookies unticked keeps you signed in.
As with other browsers, don't delete the profile folder itself; it holds your bookmarks, passwords and history.