I recently moved to a many-core system from years in a dual-core computer. When I moved, I continued my same Firefox habits such as multiple tabs but I started opening multiple windows too since I also have more screen space. However, it turns out ALL of these tabs and windows are in the same process. If I open a page that causes mass garbage collection like gmail, suddenly most of my computer's screen space is locked up. I have to sit and wait quite a while for it to unfreeze, despite 90% of my CPU sitting unused. This is unacceptable in the days of many-core. I appreciated it on a dual-core system because when firefox used 100% CPU, it really only used 50% so I could do other stuff, like run Chrome. But now I have several cores and I don't need Firefox quarantined to a single process anymore, I want it to breathe and see the rest of my processors.
Please Mozilla, if you want to compete in the real usability tests, invest more in your Electrolysis program. Some benchmarks, such as multi-tab startup, will benefit significantly from this as well as what really matters to users: usability. Frankly, I don't care about a single page's performance; 100ms versus 80ms is still in the blink of an eye. Even a full second, I can look elsewhere while Firefox loads. However, right now, I can't look at another Firefox tab while one loads because they're all slow. Therefore, I'm being pushed to the more responsive Chrome against my wishes simply because Firefox gets stuck once in a while.