Tabbed Out от Florent Ulvé

Tabbed Out unloads the tabs you are not using. Tabs that sit idle for a
configurable amount of time (1–120 minutes) are discarded, freeing the
memory they hold. When you select the tab again, it reloads exactly where
you left off — same page, same scroll position.

• Auto-unload idle tabs — a periodic sweep discards tabs that have seen
  no user activity for the configured timeout (default: 10 minutes).
• Activity detection — mouse movement, key presses, clicks, scrolling
  and touch input reset a tab's idle clock, so the tab you are actually
  using is never touched.
• Slow-reload delay — sites that historically take long to load get a
  longer idle window (×2/×3/×5, configurable) so an automatic unload never
  costs you a slow restart. Load times are measured as pages open, right in
  your browser, and kept as a rolling median per site. Sites with no
  history yet use the normal timeout, so the extension is fully effective
  from the first install.
• Pinned tab exemption — pinned tabs are never discarded (toggleable).
• Audible tab exemption — tabs playing audio are never discarded
  (toggleable).
• Site exclusions — never unload sites you choose, with simple
  hostname patterns (example.com, *.example.com, mail.*.com). Add
  them from the popup or straight from the tab context menu ("Never unload
  this site / domain").
• Activity log — a full-screen page showing what the extension did:
  every discarded tab (title, hostname, how long it idled, slow-site
  multiplier applied) and a summary per sweep. Reachable from the popup.
• Toolbar popup — enable/disable, set the timeout, toggle exemptions,
  manage exclusions, and open the activity log.

No data is collected or transmitted — no analytics, no network requests,
nothing. All state (settings, idle clocks, load-time history, the activity
log) lives in your browser's local storage on your machine.

• tabs — read tab URLs and titles to track idleness and match your
  exclusion patterns.
• alarms — the periodic sweep that discards eligible tabs.
• storage — save your settings and state between browser restarts.
• contextMenus — the "Never unload this site/domain" menu entries.
• Access to all sites — a tiny content script runs on every page to
  report user activity (mouse, keys, scroll), which is what keeps the tabs
  you are using alive, and measures page load times for the slow-reload
  delay.

• Discarded tabs are not visually grayed out in the tab strip (a Firefox
  limitation for extensions). You can set
  browser.tabs.fadeOutUnloadedTabs to true in about:config to fade
  all unloaded tabs.
• Background work pauses while a tab is unloaded: timers, open WebSocket
  connections and page notifications resume when you revisit the tab.
  Use a site exclusion for pages whose background behavior matters (webmail,
  chat, dashboards).