If I'm doing work than involves three different libraries, I'm not reading and committing to memory the whole documentation for each of those libraries. I might well have a few tabs with some of those libraries' source files too. I can easily end up with tens of tabs open as a form of breadcrumb trail for an issue I'm tracking down.
Then there's all the basic stuff — email and calendar are tabs in my browser, not standalone applications. Ditto the the ticket I'm working on.
I think the real issue is that browsers need to some lightweight "sleep" mechanism that sits somewhere between a live tab and just keeping the source in cache.