Apart from a recent security update, this was the final Java update for Leopard:
Update 10, 29 June 2011 - This release updates J2SE 5.0 to 1.5.0_30, and updates Java SE 6 to 1.6.0_26. J2SE 1.4.2 is no longer being updated to fix bugs or security issues and remains disabled by default in this update. Support for out-of-process Java applets on Mac OS X 10.5 Leopard has been removed with this update.
This release is only for Mac OS X 10.5.8 or later versions of Mac OS X 10.5. http://support.apple.com/kb/DL1359
As for plug-ins in Firefox, try this:
Open Firefox and type in the address bar: about:config (no spaces) and click return. This gives you a warning that it might harm the application. Ignore that and click on 'I'll be careful, I promise' and you get the config file that you can edit - with great care.
Look/search (scroll down) for: Firefox.plugins.enabled
Set it to true. (Double click it to toggle it)
Now close Firefox, open it again, and Flash will now work, as it now uses the plug-ins that Safari has stored.
(May not work on all versions)