Furigana inserter add-ons are nice for Japanese learners who know vocabulary based on prounciation but struggle with matching the reading with the kanji. These add-ons, on most websites, are able to insert the furigana reading of kanji right above the kanji itself!

Rikai is an in-browser pop-up dictionary that is incredibly useful for Japanese learners! Once the add-on is installed, almost any page you browse with Japanese text becomes much less daunting, as kanji you can't read can easily be deciphered simply by hovering over the mystery character.