This really depends on what is meant by the terms process and thread. They're not well defined in terms of their implementations.
Performance issues aside, which I know very little about, for IE8 it wouldn't surprise me if they were separate processes or LWPs on the basis of better reliability and isolation between tabs. Threading traditionally doesn't offer separate address spaces, which is a security hazard. Furthermore, if we're talking user threads, a crash in one will prevent scheduling of the others.
Context switching between processes is much more expensive than switching between (user or kernel) threads, but we're talking about a web browser, which hardly falls under the category of high performance computing.
This makes sense. IE8 can run one tab inside the sandbox and one outside; I guess that's why they went with multiple processes.