I still don't see the scenario you're describing, though. Let's say you have two tabs open, A and B. Then some set of pages will have memory for A, and some set will have memory for B. If you're not using A, then the operating system will notice that those pages have not been used in a while and if under memory pressure it will page them out. This won't affect pages with memory for B.
The only issue would be if most of the memory allocations are under your system's page size (typically 4096 bytes) and distributed randomly, so that a lot of pages have data structures associated with multiple different tabs. But I think that's unlikely, and even if it is true, couldn't it be resolved by making your allocation strategy tab-aware (e.g., by giving each tab its own malloc arena, which would be way simpler than splitting the browser into multiple processes).
You get it. The one thing you're missing is that application-aware allocation is hard and something that Firefox engineers continuously have to work on (http://blog.pavlov.net/2007/11/10/memory-fragmentation/), even as they add new features and performance caches. The electrolysis/chromium solution makes the problem disappear entirely (along with some security problems) without any possibility of regression.
The only issue would be if most of the memory allocations are under your system's page size (typically 4096 bytes) and distributed randomly, so that a lot of pages have data structures associated with multiple different tabs. But I think that's unlikely, and even if it is true, couldn't it be resolved by making your allocation strategy tab-aware (e.g., by giving each tab its own malloc arena, which would be way simpler than splitting the browser into multiple processes).
Am I missing something here?