The initial ESTA denials DO seem connected to the visa waiver program, and yes, work meetings are allowed under that, at least in my case, where I'm working for a foreign subsidiary of a US company, and have occasional meetings at the US headquarters. I don't know whether there is/was a Mozilla Sweden, and whether the absence of one would make a difference.

If you've never had to deal with the US immigration apparatus, think "Google customer service, backed with nuclear weapons".

One thing we get told very strongly when we travel to US for intra-company meetings is to be sure to describe them as business meetings, not work meetings. "Work" is basically a keyword for the customs official to give you a bad day.

Knowing the right words to use or not use is very useful when crossing borders (and probably when talking to powerful officials generally). One strategy is to use the least specific language you can that's still truthful.

For example, I've heard of developers get turned away at the border because they began by saying they were "speaking at a conference/giving a workshop" and noting they were being paid or having their expenses covered as if that makes it more legitimate. Instead, you'd start by saying you're "attending a conference." This is something border officials encounter 1000 times a day and will rarely question further.

Likewise, if you're planning to record a podcast or take some photos at a conference, you don't mention that unless asked specifically because otherwise it puts them on the track of treating you like a member of the "foreign media" and demanding the relevant visa for that when it's really not relevant for most people doing such things as a side effect of their attendance.

The key difference is whether you are paid in US for whatever you do. If it is a business meeting, you do not earn any income in US (your pay is in your home country). If it is work, it means that you are getting paid in US for something you are going to do here. Work visas need one of H or L categories ( or maybe something else). Business visas are the B1. The wording is ambiguous and not friendly to anyone (even to native English speakers), but that is government bureaucracy for you.

Particularly important to remember for programmers, who probably have a natural inclination to prefer to characterize what they do as "work" rather than "meetings".

Same thing traveling to the UK or Australia.

Last time I filled out the UK application, I could not find their definition of "work" or "business",

The entire thing started exactly because he was refused travel with ESTA/visa waiver back in 2017.