Can I safely deleted these .xpi files in the AppData>roaming>temp folder? Any negative consequences? They seem to be just zip files for installing extensions.
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What is the XPI file format?
File with the .xpi extension contains a data archive for installation of additional Mozilla software. The installation package is a ZIP archive compressed using the deflate method - it shares the same PK magic bytes (50 4B 03 04) as a standard ZIP file. The .xpi extension name is short for Cross-Platform Install - XPInstall, which was the name of the technology used in the original installation process.
.xpi packages are used for installing and extending the functionality of applications such as:
- Firefox,
- SeaMonkey,
- Thunderbird.
.xpi package files can contain plugins, extensions, themes, or dictionaries. The file also stores installation metadata. In the legacy XPInstall format this meant a script file install.js and a manifest file install.rdf; modern WebExtension packages (introduced with Firefox 57 in 2017) use manifest.json alongside JavaScript, HTML, CSS and icon assets. The format is closely related to JAR archives - both are ZIP containers with a specific internal layout.
Signed .xpi files distributed through addons.mozilla.org carry a META-INF/ directory containing a Mozilla cryptographic signature. Standard Firefox installations only accept .xpi files digitally signed by Mozilla. To install third-party packages not certified by Mozilla, users must use a Developer Edition of Firefox and set the option xpinstall.signatures.required to false on the about:config page. The package can be installed by dragging and dropping the .xpi file onto the browser window. Contents of an .xpi package can be browsed by changing the file's extension to .zip and unpacking it with any archive tool.
Security & safety
RISK: MEDIUMAn XPI installs code that runs inside your browser with the permissions it requests - a malicious extension can read pages, capture form data, or hijack browsing. That's exactly why Mozilla mandates signing: release Firefox installs only add-ons Mozilla has reviewed/signed, which blocks most casual malware. The risk rises when you bypass signing (Developer/Nightly editions, about:debugging) or install an .xpi from outside AMO - only do that for add-ons you trust, and read the requested permissions on install. Because it's a ZIP you can inspect manifest.json (and its 'permissions' list) with 7-Zip before installing. Unexpected .xpi files in Downloads are safe to delete; they're disposable installers.
Format details
in a nutshellFULL NAMECross-Platform Installer (Mozilla add-on package)
DEVELOPERMozillasince 1990s (Mozilla/Netscape XPInstall); modern WebExtension XPIs since Firefox 57 (2017)
CATEGORYPlugin Files
MIME TYPEapplication/x-xpinstall
TYPEZIP archive containing a browser/mail extension (WebExtension)
STANDARDOpen · royalty-free
MAGIC BYTES · FILE SIGNATURE
OFFSET
00010203
HEX
504B0304
ASCII
PK··
An .xpi is a ZIP archive (the name is the old Mozilla "Cross-Platform Installer"), so it starts with the "PK" header. Inside, a modern WebExtension XPI contains a manifest.json plus JS/HTML/CSS/icons. Rename to .zip to inspect. Signed XPIs from AMO also carry a META-INF/ Mozilla signature.
Programs that open XPI files
Windows3 apps
Mozilla Firefox
Open-source Open about:addons, then drag the .xpi onto the page (or gear menu > Install Add-on From File). Release Firefox installs only Mozilla-signed XPIs. 
7-Zip
Open-source Right-click > 7-Zip > Open archive to read manifest.json and the source (an XPI is a ZIP) - inspect before installing. 
Mozilla Thunderbird
Open-source For mail/calendar add-ons: open the Add-ons Manager and drag the .xpi onto it, then confirm install. 
macOS2 apps
Mozilla Firefox
Open-source Open about:addons and drag the .xpi onto it, or use Install Add-on From File. 
Linux2 apps
Mozilla Firefox
Open-source about:addons > drag the .xpi onto the page, or Install Add-on From File. 
Android1 app
Firefox for Android
Open-source Install add-ons from AMO within the app; manual .xpi installs are supported via a custom Add-on Collection (Firefox Nightly) - not a normal file-open flow. Technical details
deep spec| MIME type | application/x-xpinstall |
| Container format | ZIP archive; magic bytes 50 4B 03 04 ("PK") at offset 0, identical to a standard ZIP local file header |
| Modern internal manifest | manifest.json following the WebExtension API, introduced with Firefox 57 (Quantum) in 2017 |
| Legacy internal manifest | install.rdf (RDF metadata) and install.js (install script), used by the pre-WebExtension XPInstall system |
| Cryptographic signature | META-INF/ directory holding Mozilla-issued signature files; required for all packages distributed via addons.mozilla.org |
| Signing requirement | Release-channel Firefox rejects unsigned .xpi files; to allow unsigned installs, use Developer Edition and set xpinstall.signatures.required=false in about:config |
| Supported content types | Browser extensions, themes, dictionaries, language packs, and search plugins |
| Compression | deflate (standard ZIP compression); individual entries may also be stored uncompressed within the archive |
| Cross-browser compatibility | WebExtension API is shared with Chrome .crx and Edge extensions; a manifest.json extension is largely portable across browsers |
| Related format | .crx (Chromium/Chrome extension) uses a similar ZIP-based layout but prepends a proprietary CRX header before the ZIP data |
| Inspection method | Rename to .zip and open with any archive manager (7-Zip, WinRAR, etc.) to browse internal files without special tools |
| Host applications | Mozilla Firefox (primary), Mozilla Thunderbird (mail add-ons), SeaMonkey |
| Distribution channel | addons.mozilla.org (AMO) - handles hosting, review, and code-signing of all publicly distributed packages |
| Build and packaging tool | web-ext CLI - Mozilla's official command-line utility for building, linting, running, and signing .xpi files |
| Platform support | Cross-platform: Windows, macOS, Linux; also Android via Firefox for Android (Fenix) |
| Released | 1990s (Mozilla/Netscape XPInstall); modern WebExtension XPIs since Firefox 57 (2017) |
| Open standard | Yes · royalty-free |
| Specification | extensionworkshop.com |
XPI conversions
Community Q&A
asked by users
R
Frequently asked questions
How do I install an XPI file in Firefox?
Open about:addons, then drag the .xpi onto the page (or use the gear menu > Install Add-on From File) and confirm. Normal Firefox only installs add-ons that Mozilla has signed.
Why won't Firefox install my XPI (unsigned add-on)?
Since 2016 release/beta Firefox installs only Mozilla-signed extensions. To run an unsigned one, use Firefox Developer Edition/Nightly/ESR (set xpinstall.signatures.required to false) or load it temporarily via about:debugging - it's removed on restart.
Is an XPI file just a ZIP?
Yes. Rename it to .zip or open it with 7-Zip to read manifest.json and the add-on's code. The manifest must sit at the archive root for it to be a valid XPI.
Can I install a Firefox XPI in Chrome?
No. Chrome uses .crx extensions with a different format and APIs. An XPI won't load in Chrome; the developer has to port the add-on and publish a Chrome version.
How do I install an XPI in Thunderbird?
Open Thunderbird's Add-ons Manager and drag the .xpi onto it, then confirm. Thunderbird mail add-ons use the same XPI format as Firefox.
Are XPI files safe?
An add-on can run powerful code in your browser, so install only ones you trust - preferably signed ones from addons.mozilla.org. Check the permissions on install, and inspect the .xpi as a ZIP if unsure.
References
1Mozilla Extension Workshop - Publishing and signing add-onsextensionworkshop.com
2MozillaWiki - Add-ons/Extension Signingwiki.mozilla.org
