Autocomplete feature available in development environments,web browsers and search engines predicts the word or phrase based on the earlier typed words or phrases or commonly used words or phrases(in search engines).It speeds up the process of filling web forms and in logging-in to sites.As all this information gets stored somewhere.So it is possible for trojans to use this feature to gain access to your sensitive data.This feature is enabled by default in most browsers, but can be disabled.

Here are steps to disable autocomplete feature in Firefox

Open Firefox.
GO to Tools menu.
Select Options.
Go to Privacy Tab.
Go to History section and uncheck checkbox for Remember what i entered in forms and search bar.
even if you dont want to maintain history for downloaded items uncheck checkbox for Remember what i've downloaded
In order to delete History go to Private data section and click Clear Now... Button.
if you want to be prompted before deletion of private data Check ask me before clearing Private data .