This question is a slightly updated version of "Why did my computer crash?" In general people blame that question on Microsoft or the OS provider. And while there are bugs that are inherent in any application, especially an OS, the large percentage of the time, drivers are at fault that are not under the direct supervision of the OS.
So, the new update to this question is, "Why did Firefox/Safari/Chrome crash?" And, the answer could be the browser, but in my experience, more often than not it seems that plugins are to blame. Plugins like "Flash", and inline PDF readers. It seems that adding all those extras just makes the browser that much more unreliable. Maybe with the adoption of HTML 5 we'll notice crashing less as browsers implement a more streamlined browsing experience without the use of plugins.