Countdown / interval timer
05:00
Reload bookmarklet
Websites can't reload other sites' tabs, so here is the honest way: drag this link to your bookmarks bar, open the page you want to refresh, and click it. It asks how many seconds, then reloads that page on a timer until you close the tab.
Want it to keep going across reloads, with a toolbar countdown, random intervals and page-change alerts? Use the Tablark add-on for Edge and Firefox. Tablark Pro is $5 one-time.