As a power user with 50 tabs open on a tiny netbook with 1 GB RAM and 1 GHz CPU on a regular basis, this is a godsend! For the simplicity of what it does, I don't know why this isn't built into Firefox.
This worked on Pale Moon 24.0.2, with compatibility workarounds (a clean, performance-focused version of Firefox). You can use checkCompatibility or Configuration Mania (easy setting under Add-On tab) extensions to bypass the compatibility check.
I wish it had a few more options, though, like auto-load first page in background, load second page when click on first one, show unloaded tabs with page titles, ability to change unloaded tab transparency, etc.