Rated 5 out of 5 stars

The plugin can be made working for newer versions of Firefox in the following way:
- Rename .xpi file to .zip and unpack
- Within the chrome folder extract fullfullscreen.jar (with winzip) so that the included "content" folder replaces the jar file. (The folder "content" has to be there also, not just the files within.) Then delete the jar file.
-In install.rdf adapt the following line: (changing 2 to 5)
<em:maxVersion>5.0.0.*</em:maxVersion>
-Next to install.rdf add a new file chrome.manifest with the following two lines:
content fullfullscreen chrome/content/
overlay chrome://browser/content/browser.xul chrome://fullfullscreen/content/fullfullscreen.xul
- Zip up everything and rename to .xpi. Install.

Good luck.