You could try using 'Auto Tab Discard'. It interferes very little with this extension. It can discard inactive tabs (removing much of them from memory) after a certain amount of seconds. Auto Tab Discard doesn't discard loading tabs, tabs with about:blank URLs (tabs blocked from loading by this extension have about:blank URLs), or tabs with moz-extension URLs (session proof tabs). It works best if the option 'Make new background tabs session proof' is selected, unless you close firefox down after loading all the background tabs.

However, please read the 'Known Issues' before using 'Auto Tab Discard'.