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(I work for Mozilla, but not specifically on mobile.)

The reason I've heard is that the new GeckoView architecture requires piping each and every WebExtension API through a Java layer (that naturally doesn't exist on desktop). So it's some amount of manual work per API entry. The definition of the MVP only included critical extensions like reader mode and uBlock. And there's still a fair bit of other work remaining for the MVP, so nobody can spare time for noncritical tasks.

Whether that's the right definition of the MVP, and how many resources we'll be able to spend on it post-MVP (especially post-layoffs)... your guess is as good as mine, I'm going by the same public info you have.

Extensions are a pretty important distinguishing factor of mobile firefox, though.