Re: Did anybody tell them about threads?
It's also because the whole point of this is that they want to copy Chrome's security model.
Chrome has a process for each tab, and that process runs with very restricted permissions (called a "sandbox"). Those processes are managed by the main process, which also draws the browser's UI. This means that if you exploit a security hole in Chrome, you end up running code inside the per-tab process, in the sandbox. You then need a second security hole to get out of the sandbox. This makes it much harder to exploit Chrome.
By their nature, threads all run at the same security level, so you can't do that with threads.
Firefox started with a single process, and they're slowly splitting it up. Separating into 2 processes, one for all the tabs and one for the UI, is the first step. In future releases they'll split up the tabs into separate processes, then restrict the permissions.
Biting the hand that feeds IT
