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GUIDs are used in Mozilla programming for identifying several types of entities, including XPCOM Interfaces (this type of GUIDs is callled IID), components (CID), and legacy add-ons—like extensions and themes—that were created prior to Firefox 1.5. Add-ons can (and should) be identified with IDs of the form extensionname@organization.tld since Firefox 1.5.

Warning: If you just want an ID for your add-on, generating a GUID is almost definitely not what you want to do. Using the extensionname@organization.tld form is approximately one thousand times easier for everyone involved. Don't have a domain name? Do you have a blog on a subdomain? Use that. If all else fails, using should be fine; no one will care. Remember, these are identifiers, not e-mail addresses, and they're never resolved.

Canonical form

The common form of a GUID is xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx, where each x stands for a hexadecimal digit. There are a number of tools that can be used to generate a GUID in the canonical form.

Note: If you do choose to use an email-style ID for your add-on, you probably don't want it to be a real email address, since it might attract spam.

Online tools


Windows users can use the GuidGen tool from Microsoft to obtain a GUID. (This tool is also part of MS Visual C++)


Use /usr/bin/uuidgen. This can be found in package uuid-runtime (Debian).

Mac OS X

Use /usr/bin/uuidgen.


  • jkeiser's Mozilla tools include a UUID generator with output format of both C++ and IDL style.
  • sfink's update-uuids will change the uuid for a given interface and all of its descendants, updating all *.idl files within a directory tree.


A UUID can be generated from privileged Mozilla code using nsIUUIDGenerator. See the linked page for details.

COM/XPCOM format

When #define-ing IIDs and CIDs in Mozilla C++ code, you generally use the following format:

// xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx
#define NS_...ID \
  { 0xXX, 0xXX, 0xXX, 0xXX, 0xXX, 0xXX, 0xXX, 0xXX } }

You can generate code in this format using one of the following tools.

Online tools


guidgen.exe, part of Microsoft Visual Studio, can generate UUIDs in this format.


You can put the following into your .bashrc file:

    local UUID=$(uuidgen)
    echo "// $UUID"
    echo "#define NS__IID \\"
    echo "{ 0x${UUID:0:8}, 0x${UUID:9:4}, 0x${UUID:14:4}, \\"
    echo -n "  { 0x${UUID:19:2}, 0x${UUID:21:2}, 0x${UUID:24:2}, "
    echo -n "0x${UUID:26:2}, 0x${UUID:28:2}, 0x${UUID:30:2}, "
    echo "0x${UUID:32:2}, 0x${UUID:34:2} } }"


$uuid = `uuidgen`;
chomp $uuid;
print $uuid, "\n";
@parts = ($uuid =~ /^(.{8})-(.{4})-(.{4})-(..)(..)-(..)(..)(..)(..)(..)(..)$/);
print "{ 0x$parts[0], 0x$parts[1], 0x$parts[2], \\", "\n", " { ";
for (3 .. 9) {
  print "0x$parts[$_], ";
print "0x$parts[10] } }", "\n";

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