Mozilla's head of security, Window Snyder, has announced in a blog post that she will resign her position at the end of the year.
Snyder, who has the job title of "chief security something-or-other", is in charge of improving security in the Firefox web browser and other Mozilla projects.
She isn't yet saying publicly what she plans to do next. A source familiar with her plans said she is going to work at a start-up company.
"I am sad to be leaving, but I am excited to go work on something I have always been passionate about," Snyder wrote in the Mozilla security blog. "I wish I could tell you about it now, but that will have to wait for a while."
She joined Mozilla in September 2006 from Microsoft, where she was a security strategist and worked on Microsoft's security-focused Windows XP Service Pack 2 update.
Security has become more important for Mozilla as its Firefox browser has gained more users, making it a more attractive target for malicious hackers. Just last week it was being targeted by a new Trojan that tries to steal online banking passwords, according to security company BitDefender.
"It's impossible to build a perfectly secure browser," Snyder told Computerworld US in an interview earlier this year. "That's not the goal. The goal is to build the safest browser we can. It's an ongoing process. It's not a goal where we'll say, 'OK, we're done'."
In her blog post, Snyder said she was leaving Mozilla's security in capable hands, naming several colleagues who will assume her duties. She did not immediately reply to an e-mail requesting further comment.
Firefox's worldwide market share passed 20 percent in November, the first time it has stayed that high for an entire month, according to figures from Net Applications. The group also develops the Thunderbird email application.
(Robert McMillan in San Francisco contributed to the story.)