Currently, Firefox Nightly uses one process for the browser UI and a separate process for web content. However, one web content process is shared by all tabs.

Currently, Firefox Nightly uses one process for the browser UI and a separate process for web content. However, one web content process is shared by all tabs. So slow JavaScript in one tab won't block the browser UI, but it will block web content in other tabs. You can change this by increasing the "dom.ipc.processCount" preference in about:config, but there are still some known bugs with multiple content processes in desktop Firefox. There will be more work on multiple content processes in the future.