Love your extension, I found it very useful!
I have two suggestions: First, I appreciate wanting to keep the design minimalist but please add a descriptor for the buttons on mouse-over. I keep forgetting what they do.
Secondly, I would like to have an option to hide English definitions. I feel like I'm cheating a bit if the English word definition appears right there along with the kanji and I would like an option to turn them off.
Continuing from the review on AMO. I just noticed the symbols on the top right corner were buttons. I think at least on mouse over or below there should be some english to know what they do. I think the show romaji option should be right there with the rest of the buttons. It's quite hidden in my opinion now. Also I think it would be nice to have it accessible because if you have it always on, you tend to read the romaji before the hiragana and you loose on some practice,
Also I think it would be nice to have a visual cue that the kanjis are clickeable so maybe you could make the background on the area behind them look like a button or something. That would be nice IMO.