I hope you've all heard of Rikai, the browser extension that's made looking up Japanese kanji, vocabulary, and grammar online much easier in the past decade. If not, here's what it does:
Using the add-on, you're able to hover over Japanese text and see dictionary definitions through a popup. That's basically it, but it saves you from keeping other tabs open to constantly to look up new words, kanji, and grammar. Pretty useful!
Unfortunately, as new versions of browsers are released, sometimes extensions can't keep up, which seems to happen sometimes with the original Rikaichan on Mozilla Firefox.
Rikaichamp works as a lightweight version of Rikaichan (similar to the Google Chrome version called Rikaikun) that works in Firefox 57. It doesn't have all the functionality of the original, but if you've been struggling without a working Rikai, you can use this instead. But struggling without extensions like this one is a good sign you're using it as a crutch. It's best to stop using Rikai as soon as possible or not at all.
Update: RikaiChamp has been renamed to 10Ten Japanese Reader, and has immensely improved in functionality since our original writing. Check it out!