Part of the problem is that e10s Firefox by default still uses just one UI process and one "content" process, shared by all tabs. So a web page can't cause the UI to hang, but it can cause all other web pages to hang.
Curious testers can go to about:config and set "dom.ipc.processCount" higher than 1 to increase the number of content processes. I suggest trying this only in Firefox Developer Edition or Nightly, and not in your main browsing profile, since there are still some serious bugs and active development related to this configuration (so consider it a chance to test future improvements, rather than something that’s ready for regular use):