Rated 5 out of 5 stars

This plugin is just amazing!
And it works under FF 3.6 with the following trick: Download the .xpi file with Internet Explorer:
Rename it to .zip and open the zip-File. Delete the folder "META-INF" and open the file "install.rdf" with a text editor. Change the line 2.0.0.* to 3.6.*
Save the file and put it in the .zip archiv. Rename the .zip file to .xpi again and open it with Firefox. It asks you to install the addon and you are done.
The addon works smoothly with FF 3.6.3 by the way ;)
Also try the 2nd addon from this developer called "SafeCache", same method like here to get it working with FF 3.6.*