I'm having a similar issue with my Asus Transformer Prime. Running Android ICS 4.0.3. My Browser is Firefox. Flash shows it is installed but if I click to open it and get to settings I get an empty grey screen. Went through Android FAQ above and Java is enabled. I don't have a menu option to ensure add-ons are enabled that I can find. Tried uninstall, power down, and reinstall to no avail. Anything else I can try?
From what I've been told, we don't have support for Firefox on mobile at this point. Can you try with the built in browser instead?