Remember when Chrome tried to innovate with how we use bookmarks? [1]
People cry out for innovation, but then freak out when said innovation happens. This is why disrupters are so successful - they can do something crazy and people will try it out and think - this is amazing. Because the mindset/psychology is that this is something new. But the market leader implements exactly the same thing and people won't give it a chance because it's a change and they can't handle change.
But I actually disagree that they are stagnant. They may be hesitant to change the UI, but there is a lot happening under the hood to make them more secure, faster, standards compliant, stable etc.
[1] http://www.omgchrome.com/new-bookmark-manager-chrome-disable...
Edit - Adding an example of recent Firefox innovation being rejected https://support.mozilla.org/en-US/kb/hello-status