In case it helps anyone using Slackware 12. Here’s how I worked round the issue to upload images in FireFox 2.0.0.13
Upgrade Flash to the latest Linux version from Adobe – 9.0.115.0.
Extract the installer but don’t run it, it will only complain about the glibc version being too low. Edit the install bin to remove all references to glibc version checking and then run. Restart Firefox and you can now upload images.
So from my point of view, the issue is with WP using and expecting a particular version of Flash.