This page provides links to the production and development release images for the Raspberry Pi 4 bootloader EEPROM. Normally, the bootloader is automatically updated after an APT update via the rpi-eeprom-update utility. However, it's sometimes more convenient to use a recovery image to program the EEPROM with default settings for a given release, rather than updating via Linux.
Release notes are available here.
The latest production EEPROM recovery image release is 2020-09-03 and can be installed via the Raspberry Pi Imager.
USB mass storage boot requires the 2020-09-03 EEPROM images and Raspberry Pi OS 2020-08-20 or newer.
For support please see use the Raspberry Pi general discussion forum.
Old bootloader images are periodically removed from the APT package to reduce the disk space but are still available via Github here
N.B. These are not supported and may fail to boot on newer hardware revisions.