Android's app data usage is what tells me it was Firefox.
I probably overstated things with the word suddenly, but it was over the one day. And since I am on WiFi at home and at work, and it probably happened in one block of time (as WiFi data usage does not show a spike) I would have to conclude it happened within a 2 to 2.5-hour window. You do make a good point about the max rate, I've no clue what it is out there. I'll have to check if it can even possibly fit.
I was browsing over a solid half of that time, but no videos just furniture stores and some forum. And the battery did in fact drain quite fast even though I didn't think much of it on the moment. I was not throttled but just lost data access once my allowance was used up, because that's how it works on my pay as you go plan, I'm based in the UK. Of course it had to happen on day 1 of my monthly 🙃