This doesn't fix firefox's terrible memory usage.
Firefox still has memory usage of multiple GB.
When tabs are discarded, instead of data being saved to cache, the information is lost and needs to be redownloaded from the internet.
So with this addon you get a combination of bad memory usage and tabs that are basically only links. Worst of all worlds.
This is not the fault of Auto Tab Discard, it's just that firefox is written with no concern for memory and the Discard method that it exposes doesn't work usefully.