If you’ve been around a while you may remember the fantastic Rikaichan browser extension. That is no longer supported but this is a successor to it. Install this if you use Firefox to look up Japanese words with the hover of a mouse.
After enabling the add-on from the toolbar (or press Alt+R) simply hover over Japanese text and a popup displays corresponding dictionary definitions. Use Shift/Enter to cycle through the available dictionaries (word dictionary, kanji dictionary, names dictionary), and ‘d’ to toggle hiding the definitions.
Rikaichamp aims to be simple, reliable, fast, and up-to-date. It includes simple installation, an up-to-date dictionary, many many bug fixes over its predecessors (e.g. it correctly recognizes the causative passive), automated tests and type-checking, and a modern API usage.