What is the XPT file format?
Files with the .xpt extension belong to the category of plugin files for the Mozilla Firefox web browser and related XPCOM-based applications. Firefox development began around 2002-2003, though the XPCOM component framework traces back to Netscape's Mozilla codebase of the late 1990s.
.xpt files are compiled binary type libraries - they do not contain executable plugins or video codecs themselves, but rather describe the interfaces and method signatures of XPCOM components that can be added to Firefox, thereby extending its functionality. The data stored in .xpt files are binary interface definitions that allow the browser's scripting layer to call into installed components. This was the case with Adobe Flash technology, whose Firefox plugin package included .xpt files describing the plugin's XPCOM interfaces. Once installed, the browser could invoke the Flash component to play presentations and multimedia content without the user needing to manually configure the environment.
.xpt files are compiled from .idl interface definition sources using the xpidl compiler. As of Firefox 57 (November 2017), Mozilla removed legacy XPCOM add-on support in favour of the modern WebExtensions API. .xpt files are no longer shipped as separate files in Firefox's installation directory; typelib data is now compiled directly into the Firefox binary.
Security & safety
RISK: LOWXPT files are passive interface description data; they contain no executable code. However, in the context of old Mozilla/Firefox add-ons, a malicious XPT could describe interfaces for a malicious XPCOM component (the actual risk lies in the accompanying .so/.dll, not the .xpt). Modern Firefox no longer loads external XPT files, eliminating this attack surface entirely.
Format details
in a nutshellFULL NAMEXPCOM Type Library Fileaka XPCOM Typelib, XPT typelib
DEVELOPERMozilla Foundationsince ~1999 (Netscape/Mozilla XPCOM origins)
CATEGORYPlugin Files
MIME TYPEapplication/octet-stream
TYPEbinary interface description file (compiled type library)
STANDARDOpen · royalty-free
This extension is also used by…
- SAS Transport File - SAS statistical software uses .xpt (also .xpt5/.xpt8) for the SAS XPORT transport format - a completely unrelated binary format for sharing SAS datasets, defined by SAS Institute.
MAGIC BYTES · FILE SIGNATURE
OFFSET
000102030405060708090a0b0c0d0e0f
HEX
5850434F4D0A547970654C69620D0A1A
ASCII
XPCOM·TypeLib···
16-byte magic; includes CR-LF (0D 0A) to detect newline translation corruption and 0x1A to stop MS-DOS file display.
Programs that open XPT files
Windows2 apps
Mozilla Firefox (legacy, internal use only)
Open-source XPT files are not opened by users - they are loaded automatically by Firefox's XPCOM runtime during startup. Modern Firefox no longer ships or reads separate .xpt files.
xpt_dump (Mozilla build tool)
Open-source Command-line tool from Mozilla build system: xpt_dump file.xpt - dumps human-readable interface descriptions. 
macOS1 app
Mozilla Firefox (legacy)
Open-source Historical internal use only; modern Firefox does not load external XPT files. 
Linux1 app
Mozilla Firefox (legacy)
Open-source Historical internal use only; modern Firefox does not load external XPT files. Technical details
deep spec| Format type | Binary compiled type library (XPCOM interface definitions) |
| Encoding | Binary |
| Byte order | Little-endian (32-bit file offsets) |
| Magic bytes | 16-byte sequence at offset 0: ASCII 'XPCOM', newline (0A), 'TypeLib', CR+LF (0D 0A), EOF marker (1A) |
| Header structure | Magic (16 bytes) + major version byte + minor version byte + interface count + file length |
| Container | Standalone binary - not ZIP or OLE based |
| Typical file size | 1 KB - 100 KB |
| Compiled from | .idl source files via the xpidl compiler tool |
| Internal sections | InterfaceDirectoryEntry table, data pool (method signatures and type info), annotations section |
| Associated platforms | Windows, macOS, Linux |
| Format version field | major_version=1, minor_version=0 stored in header; frozen at v1.0 |
| Current status | Superseded by WebExtensions API (Firefox 57, November 2017); no longer distributed as standalone files |
| Released | ~1999 (Netscape/Mozilla XPCOM origins) |
| Latest version | Format version 1.0 (specification at www-archive.mozilla.org) |
| Open standard | Yes · royalty-free |
| Specification | www-archive.mozilla.org |
XPT conversions
Possible
Not possible
Community Q&A
asked by users
S
Frequently asked questions
What is an XPT file?
A compiled XPCOM Type Library used by Mozilla applications (Firefox, Thunderbird) to describe component interfaces. Since Firefox 57-58 (2017-2018) these are no longer shipped as separate files - the data is built into Firefox itself.
Can I open an XPT file?
Not in a normal sense - it is a binary interface description. You can inspect it with xpt_dump from Mozilla's build tools, which produces human-readable output.
Why do I have XPT files in my old Firefox folder?
Pre-2017 Firefox shipped dozens of .xpt files in its components/ directory. Modern Firefox no longer does. They are harmless leftovers from old installations.
My XPT file is from SAS - how do I open it?
SAS .xpt files (XPORT format) are completely different from Mozilla .xpt files. Open them with SAS software, R (foreign::read.xport), Python (pyreadstat), or Stata.
Is Firefox still using XPCOM?
Firefox still uses XPCOM internally for its own code, but external add-ons can no longer use XPCOM. The WebExtensions API replaced legacy XPCOM extensions in Firefox 57 (2017).
References
1XPCOM Type Library File Format v1.0 - Mozilla Archivewww-archive.mozilla.org
2Mozilla Bug 1438688 - Statically allocate all XPT databugzilla.mozilla.org
