Container Spaces от Diff

To use, just open up about:addons in a container tab and set the theme you want for that container. Container Spaces will automatically switch to that theme whenever you click on a tab with that container.

Also works great with extensions like Sticky Window Containers that let you set a container on a whole window, letting you mark the whole window with the container theme. See at a glance where all your work, social, and personal tabs are!