What evidence do you have that it wasn't tested? Or are you just making an assumption based on one article? Did you do any follow up searching? It might have led you to this:
https://www.neowin.net/news/mozilla-under-fire-for-firefox-a...
Update (August 13, 2025, 03:57 GMT): While the community correctly identified a performance issue, their attribution of the cause was mistaken.
