Firefox stores each profile you create under ~/Library/Application Support/Firefox/Profiles/, in a folder named something like <random string>.default-release. Inside it, a cache2 subfolder holds the network cache — downloaded page resources kept so they don't have to be fetched again — and a storage/default subfolder holds per-site storage: IndexedDB, Service Worker registrations, and local storage, organised the same way Chrome and Safari organise theirs, just under different folder names. The distinction matters more in Firefox than in most browsers, because Firefox's own settings let you clear these two categories separately, and doing so carelessly has different consequences. Clearing the cache is close to risk-free. Clearing site data can sign you out of everything and, for a growing number of offline-capable web apps, delete real data those apps store locally rather than merely cached copies of it.
What's actually in a Firefox profile folder
A Firefox profile is a self-contained folder, which is why Firefox can run several of them independently without interference. Looking at the sizes inside one:
du -sh ~/Library/Application\ Support/Firefox/Profiles/*/cache2
du -sh ~/Library/Application\ Support/Firefox/Profiles/*/storage/default
cache2 is disposable in the ordinary sense: it's a local optimisation, not a source of truth for anything, and Firefox rebuilds it automatically as you browse. storage/default, on the other hand, is organised into one folder per site origin, and what's inside varies enormously by site — some entries are trivial, others hold IndexedDB databases that a web app treats as its actual database, not a cache of something recoverable elsewhere.
The supported cleanup path
Firefox's own preferences, at about:preferences#privacy, under Cookies and Site Data, are the place to do this rather than deleting folders directly while Firefox is running (or even while it isn't — a profile's SQLite files and its storage folder are cross-referenced internally, and hand-editing either invites a profile Firefox can no longer parse cleanly). The Clear Data button in that section gives you two separate checkboxes: Cookies and Site Data, and Cached Web Content. Firefox keeping these as genuinely separate options, rather than one blended "clear everything," is useful — it means you can free the disposable part without touching the part that might matter.
- Clearing Cached Web Content alone is safe in essentially all cases. It empties
cache2, you notice nothing except very slightly slower first loads afterward, and Firefox refills it as you browse. - Clearing Cookies and Site Data logs you out of every site you were signed into and removes everything in
storage/default— including any web app's IndexedDB data that wasn't just a cache.
Why "just clear everything" is the wrong default here
A growing number of web applications — note-taking tools, offline-first editors, some webmail clients — use IndexedDB not as a cache but as their actual local database, sometimes as the only copy of data you haven't synced anywhere else. For those, the storage/default folder is not analogous to cache2. Treating "clear site data" the way you'd treat "empty the cache" risks losing something you can't get back from a server, because there may be no server copy.
Firefox's Manage Data view, in the same Cookies and Site Data section, lists sites individually with their storage size and lets you remove one site at a time. When you know which site is large — an editor you tried and abandoned, a project tool from a job you've left — clearing just that entry is the more careful move than reaching for the blanket Clear Data button.
Multiple profiles: the same multiplication problem as Chrome
Firefox supports multiple independent profiles, and each one gets its own complete folder under ~/Library/Application Support/Firefox/Profiles/ — its own cache2, its own storage/default, its own history and bookmarks. Power users who keep a separate profile for work, testing, or a specific client end up with several of these folders, each accumulating cache and site data independently. Clearing data inside one profile through about:preferences does nothing for the others, since that page always operates on the profile you're currently running.
You can see which profiles exist, and switch between them, through Firefox's Profile Manager. From Terminal:
firefox -P
opens the Profile Manager directly, listing every profile Firefox knows about by name. If your Firefox folder in Application Support is larger than one active profile would explain, checking here for old profiles you stopped using — but never deleted — is usually the quickest explanation.
There's a second, easy-to-miss source of duplicate profiles: Firefox's Refresh Firefox feature, used to fix a misbehaving installation, works by creating a brand new profile and copying across your bookmarks and passwords, while moving your entire old profile into a folder called Old Firefox Data on the Desktop rather than deleting it. If you've ever refreshed Firefox to fix a problem, it's worth checking your Desktop for that folder — it carries a full copy of the old profile, cache and all, and nothing removes it automatically.
A reasonable order of operations
Given the difference in risk between the two categories, a sensible routine is to clear cached web content freely and often, and treat site data more carefully:
- Clear Cached Web Content any time the browser feels large on disk. It costs nothing beyond a slightly slower reload of recently visited pages.
- Before clearing Cookies and Site Data, check Manage Data for any site you know stores something you'd mind losing, and either skip it or make sure it's backed up elsewhere first.
- If several profiles exist, repeat the above per profile, or remove profiles you no longer use entirely through the Profile Manager.
Conclusion
Firefox draws a cleaner line than most browsers between what's safe to clear on a whim and what deserves a second thought first, and that distinction is worth respecting rather than working around. The cache folder exists to be emptied; the site storage folder sometimes holds the only copy of something you'd rather keep.
We built VolumeLens because we needed this same kind of care applied to an entire Mac, not just a browser profile — showing you honestly what a folder contains before anything gets removed, and never deleting anything itself, only moving it to the Trash after you've reviewed the list. Start with a scan of your whole drive at volumelens.onzira.com and see where Firefox's profiles rank next to everything else quietly taking up space.