Works perfectly.
Thank you!

exclude pinned tabs

Simple and effective.

Awesome! Simple, effective, efficient and well-made. Thanks a lot, Dietrich! :)

works fine!

Does not require data from each tab.
This characteristic makes this much better than any alternative for Firefox 59

Update: 3/23/18 (FireFox 58.0.2 (64-bit); Dormancy 3.3.0): Dormancy has been "losing" tab content for SOME but not ALL tabs I haven't visited for longer periods (~1 day or more). When this happens, first the tab's title remains but its URL is missing (so the tab can't be reloaded), and eventually its title reverts to "New Tab."

Saves a lot of memory, no major problems with Firefox 58.0.2 (64-bit)/Windows 7 Pro/64. With current version (3.3.0), I recommend leaving "keep active window alive" unchecked. Also, the most memory is saved if I let this inactivate windows, then stop and restart Firefox (with "restore tabs at login" active). I will attempt to provide details on this to the developer in the appropriate forum.

Could you make the amount of time configurable and include a way to white list domains from the tab context menu?

Would be also nice to have on tab context menu a command to instantly unload all tabs, selected tab and tabs to the left/right