They have a common document cache, a common cookie store, a common history store, probably a common dns lookup result cache, and maybe some other things. These all need to be carefully synchronized between multiple tabs.
In fact, I would argue that multi threaded would be better than multi process so that even more things could be easily shared. For example, I imagine that css stylesheets are parsed into some big fat data structure inside browsers. If I open two tabs from a website that share a stylesheet it would be optimal if they share this same internal representation (no locking would be required for the sharing since it's read-only). This has the obvious savings of memory, but it also increases speed since the css file only has to be parsed and processed once instead of multiple times. And things like sharing keep-alive connections between tabs are virtually impossible with multi process, while very possible with multiple threads.