On desktop (Windows, Mac, Linux)

  1. Press Ctrl + Shift + Delete on Windows/Linux, or Cmd + Shift + Delete on Mac.
  2. Set “When” to Everything.
  3. Tick Temporary cached files and pages. Leave passwords and history unticked unless you want those gone too.
  4. If a login is misbehaving, also tick Cookies and site data.
  5. Click Clear.
  6. Restart Firefox and reload the page.
Firefox's Clear browsing data and cookies dialog with Temporary cached files and pages ticked
Firefox's clear-data dialog — set "When" to Everything and tick the cached files box. Image: Mozilla Support (CC BY-SA 4.0)

Tip: The long way there is ☰ (menu) → Settings → Privacy & Security → Cookies and Site Data → Clear Data.

On iPhone / Android

  1. Open the Firefox app and tap the ☰ (menu) button.
  2. Tap Settings → Data Management (iOS) or Settings → Delete browsing data (Android).
  3. Select Cache (and Cookies if needed), then confirm the deletion.
  4. Fully close the app and reopen it.

Just one site?

  1. Visit the site in Firefox on desktop.
  2. Click the padlock icon next to the web address.
  3. Click Clear cookies and site data… and confirm.