Rated 5 out of 5 stars
My favourite Firefox add-on is Rikaichan. As a student learning Japanese at a university level, I spend a lot of time reading Japanese texts online. I used to copy the kanji and words I didn't know, open a new tab, look up the Jim Breen online dictionary, paste the text, search... now, with the Rikaichan add-on, you can simply hover over the text, and BAM, there's the translation!!
Of course, this is brilliant for when you see a kanji that you don't know how to pronounce, plus it tells you right away if the word you've looked up is a name; in Jim Breen, you have to activate a separate dictionary. Furthermore, when looking up verbs that are in any form other than the dictionary form, it tells you explicitly what form of the verb it is. For example, if you look up 食べさせる, it will come up with the dictionary form 食べる(causative), which is actually more powerful than the Jim Breen website, which just directs you to the dictionary form without telling you the original verb form.
For anyone wishing to experience to Japanese online, nothing beats the simplicity of using the Rikaichan Firefox add-on.