Problem
Well, this is probably just my problem. I have been using Firefox since version 1.0, and now is 3.0. Firefox has tons of extensions, what I like the most is dictionary, because I am a language learner. And most of dictionary extensions act like a translator, you set the word and you get the meaning. If their server doesn't have the word you're looking for, you end up with nothing.
CustDict
CustDict is not a dictionary, because it doesn't have any dictionary engine. So, it cannot translate the word you wanted. However, CustDict acts like a notebook. If you find a word you don't know the meaning, you can add that word with a meaning you define. If in the future you encounter that word again CustDict will highlight it and give out the meaning. Moreover, CustDict can add any languages you want. You can mix up with Thai, Chinese, English, etc. So, this is quite useful for me and hopefully for everyone.