My experience with 50 tabs open in Firefox and Chrome is that Chrome uses about 75% more memory. You can test this yourself instead of guessing. Also, people with 50 or more tabs open represent less than 1/4th of one percent of Firefox users and I'd imagine that's roughly the same for Chrome and IE so I think it's a stretch to call that a "sensible load". 50 tabs is an extreme load and Firefox handles it better than Chrome in my experience.