I think the reason they are moving is that it is broken for them. IRC is fine if you are over forty and have been using IRC for the last few decades. I.e. it is not fine for most developers who are around 30 and have probably never encountered it professionally. The demographics of software development are such that people over forty are out numbered by an order of magnitude by younger people. There just aren't a lot of them.

I am over forty, I have used IRC professionally, and have not encountered it with any regularity in the last decade (that is to say exactly 0 times in the last six years). I can understand it's a non starter for younger developers as it is an unusable mess compared to other stuff they use. I suspect that part of the reason Mozilla is switching is that they realized people in their community were working around it already.

The real thing people should be asking about IRC is why attempts to fix, improve, or modernize it consistently get rejected by IRC users. It has not kept up and it isn't for a lack of trying. There have been many attempts to keep it alive and they all keep failing. E.g. slack had an IRC gateway for a while and this proved so unpopular that removing it was barely noticed by anyone. It was there but mostly unloved by IRC users. People voted with their feet by not using it. Now it's gone because apparently it was an unimportant/redundant/irrelevant feature with no commercial value whatsoever for Slack.