ScreenTime от Thijmen

ScreenTime measures attention, not tabs. Time is credited to the hostname of the focused tab in the focused window, and counting pauses when the window loses focus or you go idle for 30 seconds (configurable) - unless that tab is playing audio or video, so watching a video still counts.

Every site also gets an "openings" count: each time you switch to it, come back after idle, or refocus the browser counts as one distraction event.

• Popup with today's totals and a live indicator on the site being counted
• Dashboard with daily stacked-by-device charts and top sites over any range
• Optional weekly budgets per site, with an interstitial before you open a budgeted site
• Full per-day history kept locally; a compact daily aggregate travels over Firefox Sync so several devices add up

No account, no analytics, no third-party servers. If you want history beyond what Firefox Sync holds, the source repository ships a small self-hosted server you can run yourself and point the extension at - that is entirely optional and off by default.