The reason why the phones even landed in South America (and Spain) was because Telefonica heavily invested into Firefox-OS and lobbied several HW-vendors to provide devices for it.
The idea for Firefox-OS to carve-out its niche was to go lower in spec than Android could, providing a solid entry-level Smartphone experience at a price an Android device could not match just due to Hardware spec alone.
Google understood the threat, and indeed they couldn't lower the bar that low anymore and have Android provide a decent experience on a device with 1GHz single-core CPU and 512MB RAM.
As a fast measure, Google created "Android Go" as a stripped-down Android variant for lower-spec hardware, exactly to respond to Firefox-OS.
--> The fact that WhatsApp was missing on Firefox-OS was the final deathblow, yes. But even before launch of the device, the wave of "Android Go" devices matching/undercutting Firefox-OS in price was already building up, and carriers not confident in Firefox-OS were starting to order those devices.
Then, at the moment the Firefox-OS device faced its major setback after launch, that wave already came crashing down into the procurement departments of all the carriers, and so the gap to threaten Android was closed.
Just to reiterate the scale: Google made a "Go" variant of Android and ALL it's GMS-applications just to close this gap. After the threat was gone, "Android Go" was discontinued again
> After the threat was gone, "Android Go" was discontinued again
Sometimes the invisible hand of the market is just a middle finger pushing on the scale.
Normal folks just want to use phone to help them with their lives, and as such if they can’t find apps which helps them, they won’t use that phone as their daily driver.
They don’t care if it was built with rust or js or java or anything obscure.
I do wonder, though, did Mozilla ever try to convince Facebook to build a FirefoxOS build of WhatsApp?
Yes, they tried.
From what I know, a big difficulty in convincing them was that FFOS didn't have actual native applications, it was running everything on the Firefox Gecko Engine (on top of Linux). WhatsApp would have had to develop their application as a WebApp in Gecko, to run on quite low-spec hardware, which would have been a huge undertaking at that time.
(FYI, the first FFOS device was launched in 2013, Facebook acquired WhatsApp in 2014)
The problem was never technical, only business. WhatsApp was basically saying: "We'll build an app if we think we can get XY millions new users". FxOS was just not shipping enough to reach the bar. KaiOS shipped in India with Jio, Facebook saw the traffic coming from KaiOS devices using the basic FB app and decided to move forward with a WhatsApp app.
You're right, the userbase was the deciding factor, but also in context of the required effort to support the development/maintenance.
To my knowledge WhatsApp did a cost-estimation for FFOS-support back then and concluded that they need to see more than just sales projections to start working...