Here's the last few steps in my attempt to get Flash to run on Firebird.
1. I previously had Firebird installed from the .zip archive available on mozilla.com. I deleted the folder in which Firebird was installed, downloaded the installer from the texturizer site, and installed it to C:\Program Files\MozillaFirebird. My taskbar was empty at the time, and I had closed most of the programs in the systray as well.
2. I immediately opened Windows Explorer and ran the Flash 7 Player installer for Netscape, which I had previously downloaded. It showed me a list of browsers to install the plugin to. I selected "Firebird" and told it to install.
3. After installation had completed, I ran Firebird and accessed a page with a Flash object in it. Firebird did not display the object - no placeholder, no whitespace, nothing, as though it didn't exist.
4. I went to the Macromedia site and downloaded the Shockwave Player installer. I ran that too, having closed every visible program. Same problem as before.
5. Restart. Same problem as before.