For the time being, yes, they will support both. But V2 will only work on Firefox (and forks) and I think brave, a very small percentage of users.

So given that it will be like supporting two different extensions, I assume most extension developers will just switch to v3.

How long after most extensions are v3 until Firefox drops/stops supporting is anybodies guess.

It’s actually a great example of how chromes dominant position is screwing other browsers