Good news, Wacom has updated its plugin to version 184.108.40.206 and it now works again in Firefox 3.5+. So if you have that browser, reinstall the plugin with the new version and it should finally work.
I have also added a (what I consider pretty useful) feature for people who use the Wacom plugin:
You can now change what the pen pressure affects. Previously it automatically applied to the thickness of the pen. Now you can disable the pen pressure temporarily or change it to affect opacity. There is a keyboard shortcut for it too, just press the letter p to cycle through it. Thanks to Dionix and Anonymous for the idea!
Please tell me if it misbehaves, I’m not a great expert in drawing with a tablet, so I rely on your feedback