file-roller or
ark should do the trick. Typically when faced with such files on the internet, most browsers will give you a choice as to how to deal with the program (IE, whether to save it, or to open it with a program on your computer).
Admittedly, neither ark nor file-roller are plugins, but rather are programs that will unzip files outside of Mozilla (Firefox or Seamonkey). As I mentioned, Mozilla browsers could call up these programs to work upon zip files, but they would work outside of the Mozilla browsers. I'm not sure of decompression software that actually plugs into Mozilla browsers. There is a list of plugins at
http://plugindoc.mozdev.org/linux.html Perhaps something could be set up with
MozPlugger or
Plugger. Otherwise, perhaps an add-on, as markush suggested, could be found that does whatever it is you're looking for.
