Chris Blizzard, a high-profile figure in the Firefox world, has left his job as Mozilla’s director of Web platform to join a startup.

“I’m trying something new and starting in the next week or so I’m going to be joining a very small startup that’s based in Palo Alto,” Blizzard said in his announcement last week on his blog. “I happened to stumble across an amazing team that’s doing great (and difficult!) work that deals with the intersection of systems, compilers, and web-scale problems.”

Blizzard has been in charge of the “Web platform” work at Mozilla. It may sound like obscure jargon, but what it actually means is tremendously important: the upgrade of the Web from a repository of documents into a foundation for software that runs on everything from mobile phones and cars to tablets and TVs. Much of the work is embodied in the term HTML5, though in technical terms it extends well beyond version 5 of the Hypertext Markup Language standard used to describe Web pages.

Firefox doesn’t reach all these markets, but Mozilla and Firefox remain active forces in adapting Web standards so Web apps can span a tremendous range of devices.

Blizzard is a Mozilla old-timer, involved with the group since its 1998 inception, though not always employed by the nonprofit organization. He’s not the only one to leave of late. Others in recent months include:

-> Christian Legnitto, Firefox’s former release manager and a key player in the move releasing Firefox every six weeks, left Mozilla to join Facebook. However, he remains active in Mozilla now that Facebook gave him legal approval to work with Mozilla.

-> Alex Faaborg, a Firefox principal designer, left in November and now does user interface work with Google’s Android team.

-> Mike Shaver, the former Firefox technical strategy leader, now is in charge of Facebook’s Android app development.

-> Vladimir Vukicevic joined DownUnder Geosolutions after instrumental work at Mozilla on projects such as WebGL graphics and the early days of Firefox for Android.

-> Mike Beltzner, the former director of Firefox, also left Mozilla for DownUnder Geosolutions.

A certain amount of turnover is routine, of course, and these changes took place over many months. And there are good opportunities these days for people with experience in browsers and Web standards as the computing and electronics industries embrace Web apps.

But personnel changes also reduce stability at a time when Mozilla already has plenty of of transitions going on–for example, Gary Kovacs took over as chief executive a year and half ago, and Mozilla is grappling with the crucial issue of mobile browsing at top of Mozilla’s priority list.

Unlike with personal computers, where browser choice is taken for granted, Firefox is effectively barred from iOS and Windows Mobile. And on Android, Google’s browser–and eventually its new Chrome version for Android–is the one that’s installed by default. To try to carve its own niche, Mozilla has a project called Boot to Gecko, or BTG, for a browser-based operating system. For that to stand even the slightest chance against mobile OS giants Android and iOS, Mozilla needs strong Web platform features to arrive on the market quickly, so replacing Blizzard is important.

Mozilla also is dealing with the rise of Chrome, a major new Windows landscape with Windows 8, and the resurgence of Internet Explorer–at least in terms of Microsoft focus.