WHAT? That seems like a gross misuse of funds because they're definitely not spending hundreds of millions fixing bugs. They must be spending it on useless changes like the 300th UI refresh no one asked for. Firefox still sucks in every way imaginable and it's not measurably getting better. That number is shockingly unbelievable. Their crapware is not worth that budget.
For comparison, look at what the OpenBSD team has accomplished (and continues to accomplish) with a small core team and they surely do not have hundreds of millions.
Feel free to build your own browser engine that stays up to date with the current web, supports nearly a hundred languages, and runs on basically every computer with a shoestring budget. It must be easy, that's why there are so many hobby projects that manage it.
This year there was a post here on Firefox overtaking chrome in speedometer 2, so there certainly is measurable progress. Another option is to look at the interop2023 effort and how many test failures were fixed in Cross-Platform tests.
For comparison, look at what the OpenBSD team has accomplished (and continues to accomplish) with a small core team and they surely do not have hundreds of millions.