But for example you can't completely isolate a thread if it crashes. But you can forbid it to access other memory areas for security, that's what Firefox does.
Since Firefox 4 each tab has it's own separate garbage collector.
Not in C/++ in any standard definition of "thread".
Likewise for IE, but they're per process like Chrome now.
Not sure about other webkit browsers and Opera, it's likely other wekbit browsers (except safari) do not do that.