Being frugal and saving a large chunk of your income is obviously a major part of getting to FIRE, but it's pretty unlikely you'll be able to do it without making sound investments along the way. For example, someone who's 25 y.o. and whose household income is $100K might decide to save 30% of their income every year. (I'm ignoring taxes for simplicity, but the principle is the same.) So, each and every year they put away $30K into their retirement nest egg and live off the remaining $70K. This, in itself, is a pretty aggressive savings plan, one most people would have trouble pulling off. Yet, if they don't invest that $30K/year wisely, then by age 55 when they might be thinking of early retirement, they will have only amassed around $900K. Considering they have been living at a $70K/year standard for many years, this $900K will only provide about 12-13 years of living expenses. Not exactly what most people would consider a rosy or secure retirement outlook.
So, moral of the story is you need to save aggressively and make sound, consistent investment decisions to get you where you want to be. The only exception to this would be if you're making a ginormous income (say, $500K/year) and can save like 90% of that and live off the remaining 10%. If you can do that for 30 years, then investment is probably less of a factor.