Pinned tabs on demand (introduced early 2012) is Firefox's killer feature that got me using Firefox again after using Chrome almost exclusively for a year. When you pin tabs, and then restart the browser, the tabs are not loaded until you click on them (not yet loaded, pinned tabs, are visually dim).

Pinned tabs are not included in any benchmarks I've seen but have a drastic positive impact on memory usage and speed. I typically have 20 or so pinned tabs and only 1-3 unpinned tabs when I quit Firefox. A cold start takes just a few seconds and about the only time I restart the browser is when I restart my system (2-3x per week).

Chrome loads all tabs on start so it can take around a minute to load the same 21-23 tabs. And furthermore Chrome tends to operate sluggishly after a couple hours of use with so many open tabs, so when using Chrome I tend to close then reopen the browser every couple hours.

Chrome 5 had a similar feature called phantom tabs but it was removed in Chrome 6. I loved that feature and was very unhappy when it was removed.