Is dual booting macOS 9 and 10.3 possible on iMac G3?

Check out the Internet Archive. I just copied & pasted a URL contained in one of the posts in the Apple forum thread @Allan's linked and selected 2016 which had one entry for Aug. 14:

https://support.apple.com/kb/DL1283

Here is the direct link which has an archived copy of the entire Apple article which has details & instructions:

https://web.archive.org/web/20160814212149/https://support.apple.com/kb/DL1283?viewlocale=en_US&locale=en_US

I did stumble across some sites where individuals have archived some of the old software, but they are risky. I think in general they are just trying to preserve history & keep old systems alive & running, but you never know for sure.

"I recently picked up a cd copy of MacOS X Panther (10.3)..."

Do you mean a burned copy of a Panther installer disk or do you have the (Black with silver "X") retail/universal installer set of 3 CDs, designed to install on a number of compatible Apple products (at the time)? If not 3 CDs, it would be 1 DVD, similar in appearance to the disk in the photo, which the built-in optical drive in your tangerine iMac couldn't read. Attempting to use a gray-color restore disk for another model Mac (or an imaged copy of one) will likely be problematic, since the iMac requires a software build that supports its unique hardware.

As Allan mentioned, the iMac 4.1.9 Firmware Update requires it to be applied while booted from the internal hard disk that's running OS 9.1, 9.2, 9.2.1, or 9.2.2. You'll need to find a safe download source for the Mac OS 9.1 Update, and then run it to update the OS 9.0.4 your iMac is currently running. The Firmware Update may be contained in the Utilities Folder of the retail/universal installer disk, but would not be found on the restore disk for another model Mac.

I found that partitioning the hard drive into at least 2 volumes worked well for a dual-boot setup, with a separate OS installation on each volume. Partitioning requires reformatting, which erases all of the data on the drive, so you'd lose the existing 9.0.4 (OS 9.1) installation.

Is your interest in using this iMac purely for curiosity and not for use with today's internet? If the latter is your intention, you'll be disappointed. The G3 iMacs' 24~26 year-old hardware limitations and the dated software that they can run have rendered them obsolete for that task.

Finally, the convection cooling design of the slot-loading iMacs proved to be inadequate, often leading to heat-related failure of the P-A-V board that provides power to the computer. If you placed a small fan above the vent holes on top of the housing to assist with exhausting the internal heat, you may prolong its service life. When you're not using it, I'd recommend shutting it down and not putting it in sleep mode.

Correction: The original iMac DV 400 models (introduced in 1999) were manufactured in the 5 "fruit" colors and shipped with Mac OS 8.6. Your tangerine iMac DV was produced in 2000, since it shipped with version 1.1 of the OS 9.0.4 installer disk. It does have an internal DVD-ROM drive, but you may notice that it slows down or stalls when pulling in and ejecting disks. There are a pair of rubber-covered rollers inside the drive that grip the disk, but age eventually dries them out and the disk needs assistance to insert or remove. An external, FireWire DVD±RW drive is a workaround for that problem.

Wow! It's been a long time, but I recall you need to update firmware update to v4.1.9 BEFORE applying the OS update. If ignored, you get a very hard-to-fix Mac that plays dead.

You should be able to access System Profiler from your Apple menu to see the current firmware version in OS9.

In this very old thread: iMac G3 firmware download link? - Apple Community

I found the Apple download for the update, but today that link is dead, and the "Advanced" search option I used is now longer a thing. I cannot find it in the current Apple support downloads. Maybe someone else has a working Apple link and can chime in.

I cannot post a link to a non-Apple source for Apple system software in these forums, but you can do you own web search using "iMac"and the version number.

Umpteen years ago, I partitioned a G3-400 iMac SE 2000 to run both 9.2.2 and 10.4 Tiger. I had installed a larger—and apparently hotter-running—hard drive and maxed out the RAM. The lack of active cooling in the iMac made it unhappy with Tiger and/or the hot hard drive and we got thermal shutdowns. However, it did OK booted into OS9.

I suspect "Boot ROM version" shows your current fw version given the "3.24" in the value, but I need to verify. The iMacs that required the 4.1.9 updater usually had 3.X firmware versions. I did find posts on other web sites and here suggesting that you need to be at OS 9.1 or higher to apply the FW 4.1.9 updater. I need to research that more.

My PowerPC Macs, including the one the has the 4.1.9 update applied, are in storage so I cannot quickly check mine without some serious acrobatics that my geriatric body may not like. I'll try to get some more help from other senior (yes, a lot of us are just that!) contributors who remember PPC Macs.

Sorry, i'm generally new to PPC Mac systems.

Absolutely no need to apologize. I started here ~24 years ago when the PowerPC Mac was the "new hotness" and have apparently forgotten some of the minutia!

Last note, any websites you recommend to find a clean copy of the MacOS 9.1 update? I have plenty of CD-Rs laying around.

Most PPC Macs would not install system files from a burned optical disk.

Everymac.com has this: https://everymac.com/mac-answers/mac-os-9-classic-support-faq/where-to-buy-macos-9-download-software-updates-install-classic.html

They link to archive.org. I do not know if it is a safe site. The archive link works buts a slow load.

Do NOT get from CNET.