If a tab is unused for a long time, it switches to a light holding page until the tab is focused again. This helps the browser to recover memory, and can speed up the re-opening of large sessions.
I am not maintaining this script so frequently these days. Because I started to use some other approaches for auto-suspending tabs.
First:
https://chrome.google.com/webstore/detail/the-great-suspender/klbibkeccnjlkjkiokjodocebajanakg?hl=en
https://addons.mozilla.org/en-US/firefox/addon/auto-tab-discard/
Then:
I also found less need on Firefox because reopening an old session only loads pages lazily.
By the way, for managing your tabs, you could try:
