Sorry for AI but I couldn't find it by just googling:

Yes, Firefox did have an in-house developed "Tab Center" — it was an experimental feature created by Mozilla as part of their Test Pilot programme around 2016–2017.

🧪 Mozilla's Original "Tab Center" Name: Tab Center

Status: Experimental Test Pilot feature (now discontinued)

Timeframe: Circa 2016–2017

Purpose: To explore the usability of vertical tabs on the left side of the browser, instead of the traditional horizontal layout.

🔍 Key Features Displayed open tabs vertically in a sidebar.

Showed a more spacious, scrollable list of tabs.

Included favicons and tab titles — easier to manage lots of tabs.

Supported mouse gestures like tab pinning and closing.

Looked and felt very similar to Edge’s modern vertical tabs.