> The general FIRE modus operandi is to live very financially frugally for a period of time (several years or more) to save as much money as possible to, well, retire early (or at least be comfortable with a less intensive and lower paying job).

This is a subset. The only real mechanism/requirement is saving enough to retire or be financially independent, over whatever timeframe you want. That means typically means 20-30% or higher savings rate on income. One way to increase your savings rate is to have low expenses -- live frugally, as you say. The other way is to have high income.

There're several illegal ways as well.

I think that is already covered under lower your expenses or raise your income.