The focus on groups and visible thumbnails makes it easy to pick out the tabs I want visually. I'm not sure how well it holds up once you get to 100s of tabs, but for my average of around 20-40 or so (spread over several windows) it works well, and does a good job of helping to maintain context separation.
Instead, I found the Panorama/Tab Groups feature (before FF removed it, now preserved via addons like https://addons.mozilla.org/en-US/firefox/addon/panorama-tab-...) to be a better immediate fit for my workflow.
The focus on groups and visible thumbnails makes it easy to pick out the tabs I want visually. I'm not sure how well it holds up once you get to 100s of tabs, but for my average of around 20-40 or so (spread over several windows) it works well, and does a good job of helping to maintain context separation.