I’m curious why you think it was Firefox. In any situation, US based mobile networks, on a practical scale, top out around 140M per second in very robust, and not highly deployed environments. Usually those will be highlighted as 5G Ultra Wideband (Verizon’s term i think) or something. Even in the best environment, that’s still more than 1.5 hours of maximum data usage. You would be throttled back way before this point, so you’re not looking at something you did suddenly. It’s likely weeks of effort.
Are you watching a lot of YouTube, TikTok, porn, or some other video service in your browser lately? If not, then it’s not likely you’re browser. If you are, I’d recommend getting the app specific to the service you’re using. They can sometimes leverage non- standard codecs to reduce bandwidth usage while streaming. Also turn off autoplay on videos during your doom scrolls.