U

UMO (or u.m.o)

The Mozilla Extension, Theme, and Plugin web page (http://update.mozilla.org/).

UNCO

An unconfirmed bug that can't be or hasn't been reproduced by testers

URI

Uniform resource identifier. See also URL.

URL

Uniform resource locator, a type of URI. It is the address of a page or file on the Web.

Usenet

Collective term to refer to NNTP news servers.

user.js

The Firefox configuration file, intended to be modified by the user. Firefox then uses this file to update prefs.js.

userChrome.css

The Cascading Style Sheet used to configure the look and feel of Firefox or Thunderbird.

userChrome-example.css

An example of the Cascading Style Sheet used to configure the look and feel of Firefox or Thunderbird.

userContent.css

The Cascading Style Sheet used to configure the look and feel of content in Firefox.

userContent-example.css

An example of the Cascading Style Sheet used to configure the look and feel of content in Firefox.