mozilla-devscripts makes packaging Mozilla extensions easy. It provides the file /usr/share/mozilla-devscripts/xpi.mk, which can be included by debian/rules. You only have to build a .xpi file, everything else will be done by mozilla-devscripts. It will extract the xpi file, remove extra license files, install it and set the links.

debian/control

The debian/control file will normally look like:

Source: EXTENSIONNAME
Section: web
Priority: optional
Maintainer: Debian Mozilla Extension Maintainers <pkg-mozext-maintainers@lists.alioth.debian.org>
Uploaders: Your Fullname <your@email.tld>
Build-Depends: cdbs, debhelper (>= 7), mozilla-devscripts (>= 0.16~)
Standards-Version: 3.8.3
Homepage: http://example-extension.com/
Vcs-Git: git://git.debian.org/pkg-mozext/EXTENSIONNAME.git
Vcs-Browser: http://git.debian.org/?p=pkg-mozext/EXTENSIONNAME.git;a=summary

Package: xul-ext-EXTENSIONNAME
Architecture: all
Depends: ${misc:Depends}
Recommends: ${xpi:Recommends}
Provides: ${xpi:Provides}
Enhances: ${xpi:Enhances}
Description: one line for short description
 Long descriptions can span
 multiple lines
 .
 and as above empty lines

We maintain the extensions in git in the pkg-mozext group.
EXTENSIONNAME is the name of the extension. EXTENSIONNAME should not contain mozilla, iceape, icedove, iceape, firefox, thunderbird, seamonkey in it's name. Append -extension to the source package name if EXTENSIONNAME is too generic without these name (The source package name for firefoxnotify is notify-extension and the binary package name is xul-ext-notify).
You need at least version 0.16~ of mozilla-devscripts for all features mentioned here.

debian/rules

building from source

You have to run one command if you build the xpi file from source. Some extensions will provide a makefile to create the xpi file and you have to run "make", some will use ant and you have to run "ant", some will provide a shell script and you have to run "sh build.sh". Your debian/rules file will look like:

#!/usr/bin/make -f

MOZ_XPI_BUILD_COMMAND = sh build.sh

include /usr/share/cdbs/1/rules/debhelper.mk
include /usr/share/mozilla-devscripts/xpi.mk

An example for this design is notify-extension.

using xpi file

You should use med-xpi-unpack to create the source file if you use the xpi file from upstream. Please do not drop the xpi file into your sources. Your debian/rules file will be very simple if you use med-xpi-unpack to create the source. mozilla-devscripts will pack the files for you. You do not have to set MOZ_XPI_BUILD_COMMAND.

#!/usr/bin/make -f

include /usr/share/cdbs/1/rules/debhelper.mk
include /usr/share/mozilla-devscripts/xpi.mk

An example for this design is flashgot.

It is not recommended, but supported to have the xpi file in the source package. You have to set MOZ_XPI_BUILD_COMMAND to null. Otherwise the xpi file will be removed in the clean target.

#!/usr/bin/make -f

MOZ_XPI_BUILD_COMMAND := $(null)

include /usr/share/cdbs/1/rules/debhelper.mk
include /usr/share/mozilla-devscripts/xpi.mk

An example for this design is adblock-plus 1.1.1-2 (the next upstream release will use med-xpi-unpack).

plain debhelper

You can use debhelper without cdbs if you do not like cdbs. You have to call the xpi-install target to install the xpi file. With debhelper >= 7.0.50~ your debian/rules could look like:

#!/usr/bin/make -f

include /usr/share/mozilla-devscripts/xpi.mk

%:
        dh $@

override_dh_auto_build:
        # put here the commands used for building the xpi file, for example:
        xpi-pack $(CURDIR) foo.xpi

override_dh_auto_install: xpi-install

mozilla-devscripts 0.19 makes it dead simple:

#!/usr/bin/make -f

%:
        dh --with xul-ext $@

override_dh_auto_build:
        # put here the commands used for building the xpi file, for example:
        xpi-pack $(CURDIR) foo.xpi

override_dh_auto_install:
        install-xpi foo.xpi