I wanted a slight alteration, so I made my own.

Basically it scans all the paragraphs of a page, replacing 5 words with Japanese Romaji. Those words have their colour changed and are bolded. When you hover over the changed word, a small popup shows the Hiragana and the English word.

At the moment it has a smallish list of words (111), all chosen to be basic level and (hopefully) unconfusing in their context.

I'm planning on pushing the word list out to about 1000, and having it cycle through 200 every week with an overlap.

Edit: a screenshot of it in action - https://freeimage.host/i/fldb3KB

It's also uploading to the firefox addon's store, but that's taking a while.