There is a new fork https://addons.mozilla.org/addon/discard/

Work as it should

Works as it should in FF 62.
There's a slight issue tho: If you choose unload tab while the tab contents are still loading, the favicon on the tab stays the animated loading indicator until you load the tab again.

Thank you, Js, this is the only tab unloading extension that works with my Firefox, because it uses the native command browser.tabs.discard(tab.id) instead of the ported chrome.tabs.discard(tab.id) like all others.
So, to all those who have commented at other tab unloading/suspending/discarding extensions that those don’t work for them: use this one.
And to all the developers who happen to visit here: Mozilla’s online documentation for its Web Extension API specifically states the browser.xxx architecture. User it, don’t be lazy and expect Chrome porting to always work. Not all of us have Chrome Store Foxified.
Spread the word if you know any developers personally. This is important. Thank you.

Works as it should on my FF61. Unloads tab and frees memory.

Simple and functional

Finaly updated to new api, I've been expected this for so long. Thanks dev.