What these abbreviations mean? IMF is not even googleable. This is so annoying

Sorry, FMM is the forma migratoria multiplé, it's the usual paperwork for entering Mexico for any short-term reason, if you're an American (and I think some other nearby countries). It's not really a visa as those aren't required for Americans, just a form they use to track guests and mostly to collect a fee/fax for stays longer than 7 days. IMF is my own fault, I meant instituto migración federal, but that's not actually what it's called - it's the INM, instituto nacional de migración. I think I was mixing up like three different names in my head.

Mexico is really very lax about what you are allowed to do while in Mexico on an FMM and the fee for one isn't very much at all. But what you can't do is stay past the departure date on the FMM which cannot be more than 180 days from when it was issued. INM agents used to be very willing to just put the full 180 days on every FMM but lately are asking questions about your return plans and only giving you shorter time periods. Apparently there were a lot of people who were living in Mexico and basically going to San Diego at the end of their FMM, staying a few days, and then reentering on a 180 day FMM to return to where they live. This has always been against the rules but I don't think INM really cared that much until recently, because the number of people doing it increased greatly.

I'm not sure either, but I'm guessing IMF is their immigration authority, and FMM is a type of (tourist?) visa.

Costa Rica is staring to do this as well.

I am sorry, but what is the problem? So somebody already pays crazy taxes in their home country, then pays rent in another country (including taxes), sales taxes and likely spends much more than locals in the foreign economy. The foreign taxman gets a wind of possible more money, starts taxing those folks on top of what they already pay and they just move out, spread the word, weakening the economy further. If they overstay 180 days or whatever is needed for a tax domicile then yeah, they should pay taxes there, but with the current tax regimes that might not be technically possible. What prevents folks to stay 179 days in Mexico and 179 days in Costa Rica?