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The Tab Groups (Panorama) feature has been removed from Firefox 45 and above.
This has been done in an attempt to discard overheads on the core.
Luckily, the community has developed great replacements:
Tab Groups 2.0.1 No Restart
Tab Groups are an easy way to organize a lot of tabs. You can visually group related tabs, switch between groups, and quickly search through all of your tabs to switch to a specific one.