I'm not 100% sure this is a bug or not but I have the following scenario: - browser running en-US locale - extension which only provide da-DK locale I use a clean install and a clean profile so I dont have any old locale entries etc around. my XPI install.js script registers the da-DK locale but when I start the browser I'm getting an XML XUL error which complains about not being able to load stuff from the locale. if I type the chrome URL to the DTD file I'm getting a not found error. What should happen if a extension only provide and register da-DK locale while the browser runs in en-US locale? tried both mozilla latest nighly and firebird latest nightly

I think the best solution is to install the Danish locale for Mozilla resp. Firebird and switch language by Edit-->Preferences-->Appearance-->Languages/Content or create a new profile for Danish. But I think it would suffice to delete the chrome.rdf and xul.mfl (Linux: xul.mfasl), both in the profile folder, and restart Mozilla resp. Firebird. BTW, what extension is the question? Could you make the extension available?

Multi-lingual extensions have another problem: http://bugzilla.mozilla.org/show_bug.cgi?id=258725 Multi-lingual Firefox / Thunderbird habe this problem: http://bugzilla.mozilla.org/show_bug.cgi?id=259750

Bug 258725 and bug 259750 have been fixed now. Does this bug still appear ?

Not sure if this is fixed, but the Danish locale is "da" and not "da-DK" anymore.