Early retirement used to mean leaving the workforce at age 50. But a growing group of full-time workers are planning to quit even earlier.

The desire to retire in your 30s or 40s is what's driving the so-called FIRE movement. The name is an acronym for "financial independence, retire early." FIRE appears to be gaining popularity among millennials.

Two FIRE members, Tanja Hester and her husband, say they both had demanding consulting jobs. Each earned more than $100,000 a year, but both careers required long hours and extensive travel.

"My husband and I had jobs that we loved, actually," Hester told CNBC's "On The Money" in a recent interview. "I think there's a common misconception that those of us pursuing early retirement are doing it because we hate work. Or we feel like we shouldn't have to work. It really couldn't have been farther from the truth for us. It was just that we felt the toll that work was taking and we knew we couldn't sustain that pace until age 65."

Six years ago, they began to envision a life without daily 9-to-5 jobs.

Hester said they started "saving more without a real plan, then we built out a detailed plan." That strategy included investing more of their income, while cutting spending.

Simultaneously, she started chronicling their progress on a blog, "Our Next Life." It's one of many FIRE movement blogs and podcasts aimed at, for and about the group by those involved in it.

The FIRE movement is "a program of extreme savings and investment that allows proponents to retire far earlier than traditional budgets and retirement plans would allow," according to Investopedia.

While sharing similar goals, Hester said the methods to get there differ.

"Like any group of people, you've got a wide range of folks in the FIRE movement." she told CNBC. While some focus on extreme frugality, Hester said she and her husband don't share that mission.

"Mark and I are much less frugal. We weren't natural super-savers. And I wanted to show folks even if you don't love counting every cent you spend on your groceries, you can still pursue early retirement or some form of it. "

Although she declines to share how much they accumulated in their portfolio before quitting work, they officially began their early retirement last year. She at 38 and her husband at age 41.

"Early retirement isn't magic, it doesn't make life perfect. But I do like no alarm clock." She said in their first year, "a big chunk of the year we were away traveling," to destinations including Taiwan, Mexico, France and Monaco.