This was so harrowing an experience that I thought I would document it here for those who may be searching for solutions to this problem. My own searches yielded no results which could just mean that this problem was peculiar to me (and was caused by some meddling on my part!). Keep in mind the usual warnings. Now on to my experience…

I have Mac OS X 10.3.9 (Panther) on my PowerPC laptop and decided (for reasons not worth going into here) to install the latest version (9.0) of the Adobe/Macromedia Flash Player. The player can be downloaded at this page on Adobe.
The problems start right away. The correct option for my platform is Option 2 (PowerPC-based Macs) on the above page. However, there are two links under that section. One is an image with a downward pointing arrow (to signify “download” I presume) and the other is the text “Download .dmg file”. Problem: they point to different binaries! One (the arrow) points to a 7.0 binary (a Mac Classic one, I think) while the DMG link points to the right binary (9.0.x.x). There is a star next to the “Download .dmg file” link, but it seems to point to nowhere.

Fortunately, I did download the 9.0.x.x DMG. The install went well, but when I relaunched Firefox and Safari, the former displayed a blank box where flash content would be, while the latter displayed an error intimating that the Flash player plugin could not be loaded. A look at the console log showed that a necessary file was absent in /Library/Internet Plug-Ins/Flash Player.plugin.

Next step: the usual uninstall/reinstall attempt. I downloaded the uninstaller from Adobe… well, there are two uninstallers linked to in two separate pages. The first one froze my computer requiring a hard reboot (probably my impatience after waiting for 10 minutes is somewhat to blame). The second did the trick and carried out the uninstall. A re-install however made no difference.

Time to get dangerous: I carried out one more uninstall, followed by removing all the flash related files and folders in the above-mentioned plugins directory. An attempt to re-install produced a Permission Denied error. I have seen that one before in a different context! Onwards to the Disk Utility to select my main disk and run Repair Permissions on it.

That did the trick. Re-installing from the 9.0.x.x DMG brought Flash back to life on both Safari and Firefox.