Got a new computer. Had several versions of Java preinstalled. Installed Firefox myself, Java applets didn't work.

Tried this, tried that, even installed a new JRE myself. No success.

Used procmon and found out that, after enumerating the HKLM\Software\Wow6432Node\JavaSoft\Java Runtime Environment keys, firefox tried a RegCreateKey on HKLM\SOFTWARE\Wow6432Node\mozilla.org\Mozilla which failed with "Access denied".

Started firefox with admin rights. Java worked.

Changed the permissions on HKLM\SOFTWARE\Wow6432Node\mozilla.org\Mozilla to "full access everyone" and restarted firefox without admin rights. Java worked as well.

Don't know if that's a FF5 problem, or if the pre-installation of the computer broke something, but anyway - firefox shouldn't behave like that imo.