What Is an XPI File?

XPI files are compressed installation archives used by Mozilla software

What to Know

  • An XPI file is a Mozilla installer package. It might contain a browser plug-in or theme.
  • Open one with Firefox through the Add-ons Manager screen, or drag the file into the browser.
  • For Thunderbird, go to Tools > Add-ons.

This article explains what XPI files are, which apps use them, and how to open one in each respective program.

What Is an XPI File?

An abbreviation for Cross-Platform Install (or XPInstall), a file with the XPI file extension (pronounced "zippy") is a compressed installation archive used to extend the functionality of Mozilla products like Firefox, SeaMonkey, and Thunderbird.

An XPI file is really just a renamed ZIP file the program can use to install the add-on, whether it be a theme, plug-in, etc. They may include images and JS, MANIFEST, RDF, and CSS files, as well as multiple folders full of other data.

xpi files used by Firefox

How to Open an XPI File

Firefox uses XPI files to provide extensibility in the browser. Drag the file into any open Firefox window to install it (select Add when you see the prompt).

Mozilla's Add-ons for Firefox page is one place you can go to get official XPI files to use with their browser.

Another way to add an XPI file to Firefox is through the Add-ons Manager screen. From the menu at the top-right of the program, choose Add-ons and themes. Next to the Manage Your Extensions heading, select the gear button and choose Install Add-on From File.

Firefox Add-ons Manager install from file option

Add-ons for Thunderbird provides XPI files for their chat/email software, Thunderbird. These files can be installed through the program's Tools > Add-ons menu (or Tools > Extension Manager in older versions).

Although they're now discontinued, the Netscape and Flock web browsers, Songbird music player, and Nvu HTML editor all have built-in support for XPI files.

Since XPI files are really just .ZIP files, you can rename the file as such and then open it in any archive/compression program. Or, you can use a program like 7-Zip—right-click the XPI file and open it as an archive to see the contents inside.

If your file isn't used by a Mozilla program, but you don't know what else it could be, try opening it in a text editor. If the file is readable, then it's simply a text file. If you can't make out all the words, see if you can find some sort of information in the text that can help you determine what program was used to create the file, which you can then use to research a compatible XPI opener.

If you're wanting to build your own XPI file, read more about that on the Mozilla Developer Network. Another helpful source are the users at StackExchange.

How to Convert an XPI File

There are file types similar to XPI that are used by other web browsers to add extra features and capabilities to a browser, but they can't be easily converted to and from other formats for use in another browser.

For example, although files like CRX (Chrome and Opera) and SAFARIEXTZ (Safari) can all be used as add-ons to each respective browser, none of them can be used in Firefox, and Mozilla's XPI file type can't be used in any of these other browsers.

However, there is an online tool called Add-on Converter for SeaMonkey that will attempt to convert an XPI file compatible with Firefox or Thunderbird into an XPI file that will work with SeaMonkey.

If you want to convert XPI to ZIP, keep in mind what we mentioned above about renaming the extension. You don't have to actually run a file conversion program on it to produce a ZIP file.

Still Can't Open the File?

Lots of files use three letter extensions, so you're bound to run into files that use a similar extension. The problem with this is that it's easy to confuse them for each other, leading you to open an incompatible file with one of the programs on your computer. This usually leads to errors or strange looking files if they do happen to open.

XPI files use an uppercase "i" as the last letter of the file extension, so don't confuse them with XPL files that use an uppercase "L"—these are LcdStudio playlists. Another similarly named file extension is XPLL, which is used for Pull-Planner data files.

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