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.