The simple story is that the current tabbing model in was designed to save you from needing half a dozen browser window open. It just doesn't scale well past a dozen tabs or so, and we know that some users have dozens or even hundreds open at the same time. Tab Center is taking a fresh look at the problem with that in mind.
The whole problem of a tab UI for power users with hundreds or even thousands of open tabs (like me) is mindblowingly daunting. I don't know that tabs on the side is the solution. For a long time I'd been using Tab Groups but I've been frustrated with its limitations.
Ideally, I think I'd like something similar to the spatial system of the Classic Mac OS Finder, which I coincidentally expounded upon recently in another thread[0]. Tab Groups goes part of the way there with its spatial organizational system but it lacks the hierarchy and persistence of those Classic folders and desktop.
Another problem is that it's hard to experiment in the browser; the stakes are very high when bugs or poorly-conceived ideas quite literally break the internet for users. As a result, the development cycle is righteously slow (~18 weeks), the cost of landing code in Firefox is righteously high, and it's just a difficult environment to try new things.
On Test Pilot we aren't trying to build the solution, we're trying to find it. It's a different model of product development than we've historically had at Mozilla. In this case, side tabs are the start, but you can expect new features and concepts to start coming down the pipeline as we start gathering feedback and data about how people use them.
My issue with systems like this is that they consume valuable horizontal real estate. Like most people, I use a widescreen monitor. To take full advantage of this, I use a tiling window manager (Xmonad) and typically use two browser windows side-by-side, akin to a full two-page broadsheet newspaper. Putting a tab bar on the side of each window would consume nearly 1/3 of the screen and generally get in the way of the content.
I do hundreds of tabs. The UI is fine, but performance is not at all OK.
The tab UI works because it is part of a larger 3-level system. There are tabs, windows, and virtual desktops. With 10 at each level, you can handle 1000 tabs... except for the performance.
It's important to avoid running things on pages that aren't in focus or even in view. It's important to avoid walking data structures that scatter nodes all over the address space, causing swap access and cache misses. Watch your RSS. Keep those extra tabs idle. Make sure the "Esc" key and the stop button actually work, stopping everything (all tabs, all video, all animation, all audio, etc.) until the user explicitly asks for something to run again.
The simple story is that the current tabbing model in was designed to save you from needing half a dozen browser window open. It just doesn't scale well past a dozen tabs or so, and we know that some users have dozens or even hundreds open at the same time. Tab Center is taking a fresh look at the problem with that in mind.