Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes)

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

You can use "Disable all add-ons" on the Safe mode start window to disable all extensions.

You have to close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

Still does not work in safe mode. I even tried after disabling all add-ons and then just enabling Flash. Still no luck.

the exact same problem here, but on windows xp sp3.

the other system/software setups are exactly the same as those of crashd0g. firefox 3.6.11, flash 10.1.85.3. doing a complete, clean reinstall of ff, or starting ff in safe mode with only sf enabled and other add-ons disabled has not solved the problem for me either. regardless of which website i visit, flash does not load. to add, the problem did not start to occur on 3.6.11, but on 3.6.10.

visiting the adobe sf testing site doesn't even tell me which version of sf ff has, nor does it tell me to install the latest version of sf. the said site shows only black or gray spaces at where it is supposed to show "congrats, you have successfully installed sf", or "you have the latest version of sf".

Do you have a firewall or other security software that can block Flash?

You may also have a problem with OOPP.
Make sure that your firewall doesn't block the plugin-container process.

See http://kb.mozillazine.org/Plugin-container_and_out-of-process_plugins

The windows firewall is disabled. Other systems can load flash objects, so not likely an external firewall. I'll try to rebuild the whole machine to see if that will clear up the issue. Not a great solution if it works, but I'll try it.

Also, same machine can load flash objects in IE 8 (not my preferred, but works for now).

So far, the only solution I have found to the problem is to add the flash object with the embed tag included. The embed tag is inferior in a few ways, such as it adds a small space after it, but it works in Firefox and Internet Explorer.

If you see a small space after a Flash object then make sure that you add a CSS rule {display: block} to that object (embed).

Well, for me at least, I think an upgrade was the problem. I have re-installed everything on the machine (including OS) and don't have the problem this go around. Kind of an extreme approach, and not for everyone, but not too big of an issue for me. I'm still not sure what the problem was, but for me after the complete rebuild of the system the problem is gone.