Firefox's fall is notable as the market is quickly becoming a two-horse race. Chrome has always been the fastest and safest of the brand name browsers and it appears as more and more people buy Android devices, Chrome and its syncing ability is becoming more popular. Firefox on the other hand, has been losing fans ever since they decided to move away from large annual updates to nightly and more regular background updates, just like Chrome. The change was confusing, and Mozilla has not been great with advertising new or exciting features. Most of the time, it seems like the devs are playing catchup with Chrome or Safari.
Speaking of Safari, Apple's browser continued to be stagnant (at 5% share) as Mac sales are relatively stagnant, as well.
Check out the rest of the numbers here.