I first found this plugin when it was called “perapera kun” and it required you to find a japanese dictionary plugin for it to work. Now I think they’ve updated the plugin to the point where anyone can immediately get started without needing any extra downloads and I recommend checking it out if you’re interested.
What does it do specifically? Well I’m still finding all of that out myself. One of the things it does is to simply allow you to highlight words in Japanese and it gives you the reading, meaning and a bunch of other information. In a few cases it even gave me several sources to look up the word including the Kanji Learner’s dictionary and the SKIP number.
I think they’ve got a version out for Chrome as well now in case you’re not using Firefox. The website is http://www.perapera.org/