Default stylesheet
Each browser has a default stylesheet. Accepting the presentation defined by the default stylesheet can simplify your job designing your pages. However, different browsers (IE, FireFox, Opera, SeaMonkey, etc.) have different default stylesheets, which can sometimes make it hard to get consistent appearance across multiple browsers.
The FireFox default stylesheet can be found at
C:\Program Files\Mozilla Firefox\res\html.css
It can be helpful to inspect it to understand the default look, but do not modify it. Doing so could prevent FireFox from displaying pages intelligibly.
Reset file
Many Web designers create "reset files" to override the default styles, with the objective of having more control of how their pages look across multiple browsers. The reset stylesheet is then the first stylesheet linked to on every page the designer creates, hopefully creating a uniform foundation for all the designer's other stylesheets.
More information
- Really Undoing html.css - Eric Meyer
- YUI Reset CSS - Yahoo User Interface library