XPI (Firefox Extension) file format

.xpi · package (signed ZIP) · introduced 2002 by Mozilla

What is a XPI (Firefox Extension) file?

XPI ("Cross-Platform Install") is the Mozilla extension package format. It is a regular ZIP containing a manifest.json (WebExtensions) plus extension code. Mozilla signs every public XPI; Firefox refuses to install unsigned extensions outside the Developer and Nightly channels.

Most readers reach this page after double-clicking a .xpi file and finding their operating system unsure what to do with it. The good news: XPI (Firefox Extension) is a well-understood format with mature tooling on every major platform. It is supported in one click by mainstream archivers on Windows, macOS, and Linux.

How to extract a XPI (Firefox Extension) file

Opening a .xpi file is straightforward on every modern operating system. The exact steps depend on which platform you are on:

On Windows

Locate the .xpi file in File Explorer.
Right-click the file. If you have 7-Zip installed, choose 7-Zip → Extract Here. On Windows 11 the built-in extractor handles ZIP, 7z, RAR and several other formats directly through Extract All…
If prompted, pick a destination folder and click Extract.
The extracted files appear in a folder next to the original archive.

On macOS

Find the .xpi file in Finder.
Install The Unarchiver from the Mac App Store, or Keka for both creating and extracting.
Right-click the file → Open With → The Unarchiver (or just double-click after Unarchiver is set as the default).
The extracted folder appears next to the archive.

On Linux

Use your file manager: right-click the archive → Extract Here (file-roller on GNOME, Ark on KDE).

On the command line, run one of:

7z x archive.xpi    # works for almost any format

The files appear in the current directory.

How to create a XPI (Firefox Extension) file

To pack files into a .xpi archive, you typically need an archiver that supports writing this format. Not every tool can create every format — for example, only WinRAR can create real RAR files. The sidebar lists the software known to read and write XPI (Firefox Extension).

On the command line, the canonical create command is:

# Use 7-Zip, PeaZip, or the format's reference tool to create .xpi files.

If you regularly create archives at scale, a dedicated archive automation suite can wrap this command in scheduled jobs, integrity checks, and offsite replication.

Strengths

Just ZIP
Cryptographic signing
WebExtension cross-browser-friendly

Weaknesses and limitations

Signing required for general distribution

Typical use cases

Firefox extensions
Thunderbird add-ons

Technical details

XPI (Firefox Extension) uses the ZIP + Mozilla signature algorithm and is identified on disk by the magic byte sequence 50 4B 03 04. The standard MIME type is application/x-xpinstall. The format is an open specification and is free for any use.

Frequently asked questions

Is XPI (Firefox Extension) safe to open?

Archive files are containers — they are only as safe as what they contain. A .xpi from a trusted source is fine; one received from a stranger can hold malicious executables, scripts, or files engineered to exploit bugs in your archiver. Always keep your archiver updated, and never run unknown executables that come out of an archive. A safe workflow is to extract into a sandbox and scan the contents before opening anything.

What is the maximum file size?

Modern ZIP-based formats use the ZIP64 extension to support archives larger than 4 GB and individual files larger than 4 GB, up to 16 EB in theory. Older tooling that does not understand ZIP64 will report a corruption error on such files.

Can I password-protect a XPI (Firefox Extension)?

Some archivers can wrap XPI (Firefox Extension) files in an encrypted container, but the format itself does not always include built-in encryption. Check your archiver's documentation.

How does XPI (Firefox Extension) compare to other formats?

See our format comparison pages such as ZIP vs 7z, ZIP vs RAR, and tar.gz vs tar.xz for side-by-side feature tables.