What do you think about Founder-Market FIt?

I think it's absolutely important. I think it's naive to think that you can just enter into a market you know little to nothing about and then somehow thrive and beat incumbents in the space; it's ridiculous. Startups are already very risky. I'd remove risk by sticking to what I know AND like.

If you're not at the leading edge of some rapidly changing field, you can get to one. For example, anyone reasonably smart can probably get to an edge of programming (e.g. building mobile apps) in a year. Since a successful startup will consume at least 3-5 years of your life, a year's preparation would be a reasonable investment. Especially if you're also looking for a cofounder

by How to get startup ideas

If you define yourself as a freelancer, then you are already in the freelancer community and part of the freelancer market :).
Asides I believe a lot of very valuable business are "simplify" adapting an already existing product/business model to either:

1. A local market - Outside language, banking and payment processing preferences vary greatly from country to country.
2. A different industry - Like in Mathematics where you can transform a geometry problem into an algebraic problem, prove an algebraic theorem and conclude on the geometry problem, there is a lot transferable knowledge from one market to the next.