Browser plugins can add functionality to the game right in your browser where the games is. Please find some useful examples below:
One of the advantages of Firefox is that add-ons can be created quit easily. Due to this, a lot of add-ons ARE beging created. One of the add-ons I use for Hattrick is PsicoTSI.
Foxtrick is a browser extension for hattrick. It enables the usage of skins in hattrick. Foxtrick makes calculations and creates extra shortcuts within Hattrick. In short: it makes external Hattrick Tools available right into the user interface of hattrick.