What is a .xpi file?
Mozilla Cross-Platform Installer (Firefox Add-on) · Archive · Mozilla · 2000
An .xpi file is a Firefox (and Thunderbird) extension package: a ZIP archive holding the add-on's manifest, scripts and icons. Firefox installs it directly; everything else can simply unzip it.
What is a .xpi file used for?
XPI (pronounced 'zippy' at Mozilla) dates back to the Netscape XPInstall system. A modern WebExtension .xpi contains a manifest.json plus JavaScript and assets, and must normally be signed by Mozilla before release-channel Firefox will install it.
Because it is plain ZIP under the hood, renaming to .zip and extracting shows exactly what an add-on does, a useful transparency check before trusting one.
How to open a .xpi file
The .xpi format opens in the following apps, grouped by operating system. If one app does not work, try another from the same list. For a step-by-step walkthrough, see how to open a .xpi file.
Windows
- Firefox (installs it)
- 7-Zip (inspect)
macOS
- Firefox
- The Unarchiver (inspect)
Linux
- Firefox
- unzip (inspect)
How to convert a .xpi file
You can convert a .xpi file to zip (rename) using a conversion tool or the export menu of an app that opens it.
.xpi file signature (magic bytes)
Programs recognise a .xpi file by the bytes at the start of the file, not by its name. These are the signatures for XPI. For a full breakdown, see the XPI file signature page.
| Hex signature | Offset | Note |
|---|---|---|
| 50 4B 03 04 | 0 | ZIP package (PK..) |
See the full magic-numbers reference for every format.
Frequently asked questions
What is a .xpi file?
An .xpi file is a Firefox (and Thunderbird) extension package: a ZIP archive holding the add-on's manifest, scripts and icons. Firefox installs it directly; everything else can simply unzip it.
How do I install an .xpi file in Firefox?
Drag it into a Firefox window or open it via the menu. Firefox verifies Mozilla's signature and asks for the extension's permissions before enabling it.
Why won't Firefox install my .xpi?
Release Firefox refuses unsigned extensions. Get the add-on from addons.mozilla.org, or use Firefox Developer Edition/Nightly if you are testing your own unsigned build.
What is inside an .xpi file?
A ZIP archive with manifest.json describing the extension plus its JavaScript, HTML and icons. Rename to .zip and extract it to look.
Related formats
Sources
Details on this page were checked against authoritative references: