Most programmer's text editors and IDEs have it, automatic completion of the first few characters (letters) of a keyword or function name. Also, most address bars or search fields feature this (like the tag input below the posting form). I would like the same for a few, often-occuring words in business correspondance. Since I could not find an extension for the application in question, Mozilla Thunderbird, I would like to know if there are applications or methods that suggest the complete words from a dictionary like

sup
   per
   eruser
   erfluent

PS: Here's an article on Wikipedia: Word completion

Here's a product for $25: Type Booster

Ideally, the solution is free.