The fact is, browsers are fickle things and when you combine that many different web sites doing all that stuff inside one process even tiny little bugs or small, rarely triggered memory leaks really start to build up and cause problems. Of course it's a security risk, but that doesn't mean it doesn't happen or even that it only happens rarely. You might think that web sites can't crash your browser – that would be a security risk right? At least a denial of service, if not a possible remote exploit. Then they all just go away in the blink of an eye and you have an excuse to go get another cup of coffee and slack off for a while. That is until one of them hits a browser bug.
You get to load up all these different applications, email, word processing, spreadsheets, encyclopedias, music players and photo software – all living happily in harmony. That's right, your browser has it's own OS 9 experience built right in. Not a complete emulator, but it reproduces that critical "me time" preserving feature of having everything in the same memory space. They'll never admit it, most of them don't even realize it, but nearly everyone runs an OS 9 emulator on an almost daily basis. Everyone longed for the opportunity to sit back and relax while everything rebooted. Of course, deep down everybody loved OS 9 – even those who'd never used it. People rejoiced as they left their computers running for days on end without needing reboots and uptimes soared. At last OS X came along and solved all that, finally providing protected memory and the ability to force quit one application without affecting the others. Probably the most commented on experience from Mac OS 9 was the fact that once one application crashed, they were all in trouble generally, you were in for a complete system reboot.