Changes to how extensions are installed in Firefox 4
The next beta version of Firefox 4 will contain changes to the way that Firefox installs and uses extensions. Previously, files taken from an XPI file of an extension were unpacked into their own directory (in the file system) and read from there, and now by default, the XPI will be saved simply, and the files will be read from it directly.
For many extensions, this change will not bring problems; However, if the code for your extension tries to read its files directly, or if the extension contains binary code, or search plugins, or dictionaries, or window title icons, then you will have to either make changes to the extension that support the new installation method, or request that Firefox continued to unpack your extension files using the <em: unpack> directive in install.rdf.
Learn more about this change and about ways to support it in its extension, described on the page " Firefox 4 for Developers ".