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Google to push ahead with Chrome's ad-blocker extension overhaul in earnest
(www.theregister.com)
This is a most excellent place for technology news and articles.
I think Firefox will support both v2 and v3 extensions, so devs can use whichever makes more sense for their project. It has been a while since I looked into it though.
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