Filling forms again and over again is a quite tedious task. There are Firefox add-ons to cover this job but the downside is that such proprietary addon is limited to one particular program. Any stored abbreviation would not be available anywhere else (e.g. if you switch to another browser or if you work in another program).
The text expander PhraseExpress works system-wide in any Windows program and allows you to quickly fill in forms in your favorite web browser as well as in any other application such as a PDF form, etc.
Video demonstratin (turn on speakers):