However, your computer's BIOS, while in the past was usually impossible to change, can today be upgraded easily. That's why we now have Coreboot.
You make it sound as if technical difficulty in changing BIOS software is the issue, and I'm not sure if you realize that is not so. Software freedom has to do with an ethically based argument about giving permission to legally inspect, share, and modify published software (and, ideally, securing those freedoms to make sure nobody takes them away later). With BIOS code, as with any proprietary software, the distinction is not technical capability. The distinction centers on who is legally allowed to do what.
BIOSes prior to the arrival of Coreboot weren't all "impossible to change". BIOS distributors demonstrated that by making BIOS changes and distributing new proprietary BIOS software packages. Users were offered proprietary binaries that they could run—an ordinary installer program that allows non-technical users to easily install a new BIOS on the system.
But users were not given a copy of BIOS source code, users were not given permission to distribute the BIOS, and users were not given permission to modify the BIOS software. These users were subjugated to the BIOS developer's rule so long as they ran that BIOS code: users had no freedom to help themselves or their community. Coreboot changes this because Coreboot respects a user's software freedom, but the difference here is one of licensing. With Coreboot any user willing to learn what Coreboot does may inspect, share, and modify Coreboot; freedoms those same users don't have with a proprietary BIOS.