I think that was definitely the impetus - I first read about the changes in this article back in April: https://www.theregister.com/2023/04/18/reddit_charging_ai_api/
The closing statement is interesting:
The spokesperson we talked to also wanted to make clear the Data API was still freely accessible for appropriate use cases through the Reddit developer platform; hopefully app developers and other small-scale operators won’t have any surprises ahead this summer.
I suspect they ran the numbers and started seeing dollar signs - they don’t care about the third-party apps (which don’t make them any money directly), they’re just trying to cash in on Microsoft etc.
I have a sneaking suspicion they’re going to end up back-pedalling, but it will be too little, too late.
You can still do this in Waterfox, FYI.
I’ve not used Firefox in years (just various forks), specifically because of Mozilla’s insistence on changing the UI and not allowing you to change it back.