Long time ago, i had tried on an extension call 'LeetKey'. It might be lame for the name itself and some of the features it provide, but there are some functions inside look interesting to me.

Some features included:

Inside the 'Toogle Editor', other it allow me to type in L33T way (~_~, it also allow to directly type in binary, hexadecimal, morse, reverse way....

Under 'Text Transformer', it allow to all type of encode-decode things (base64, Hex, Bin,etc...) that is useful for me when i have intercept some request from 'Cookie Edition' or 'Tamper Data' (another 2 extension in Firefox) and to view the plain text on the spot~!

On the 'Text Encryption', it also allow me to encrypt & decrypt DES~ (although DES might be useless these day >_~)

Overall, this is an extension worth to play around with~ ^_^