When the original iBook in Tangerine and Blueberry was presentesd by Steve Jobs on July 21 in 1999 it came with Mac OS 8.6. Only a couple of months later in October the new OS9 was released. According to Apple's product policy all new hardware turned out from that point need OS 9 as minimum OS.
Clamshell iBooks with 300mhz were produced until September, so it is likely that your Tangerine or Blueberry will refuse to boot OS 8.6.
If you want to prove if you Clamshell will boot OS8.6 or not, take a look at the ROM Revision presented by the Apple System Profiler. If the version is $77D.45F3 you can install MacOS 8.6 - whether or not Firmware update 4.1.7f4 is already installed or not. You will find this ROM Revion also on several iMac, PowerMac G3 and G4 computers released until late autumn 1999.