because there's a lot of proprietary stuff in microcode that's used for accelerations. gfx drivers too. it's the reason the closed amd drivers are so much faster than the open mesa ones.
yeah... I don't know what numbers you're looking at but that's not true in the general case. and this isn't firmware, it's microcode. firmware is already on the chip. microcode is used so the os can take advantage of chip specific features, like security patches or even acceleration.
Do you have actual benchmarks which show the closed source OpenGL driver significantly faster than the open source one? In Phoronix benchmarks I've seen, the open source driver beats the closed source one by a large margin.
A lot has changed in the last two years. Nowadays you have an occasional game that is faster on the blob driver, but most are faster under Mesa, often significantly so.
Mesa almost always uses proprietary firmware. The fail0verflow guys did some work last year to at least document it for the PS4's GPU to patch a bug. But the upstream Radeon Mesa guys are really hesitant to upstream it to avoid pissing off AMD. https://github.com/fail0verflow/radeon-tools/tree/master/f32
Of course that's all sorta orthogonal because that's all not really microcode or firmware in the classic definition, but just "code for an embedded processor I don't want to document."
On the contrary, Mesa is faster than their blob. AMD themselves are working on replacing blob with Mesa in the long term.
Firmware doesn't offer any acceleration advantages, it's used for different purposes.