When I installed, all tabs already discarded were loaded. This is fatal.
I recommend extension like "Auto Tab Discard" instead of this.

Enabling this add-on activates all currently unloaded/discarded tabs. Firefox immediately tried loading my 200+ unloaded tabs, effectively breaking my session. This add-on also breaks firefox's default restart behavior which is not to load background tabs.

Seems to work, but will also break middle-click / ctrl-click on a link: a tab will open but it won't load and the url is lost, so one can't re-load ):