User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de-DE; rv:1.7) Gecko/20040803 Firefox/0.9.3 Build Identifier: TThunderbird 0.8 Note: This is not a Thunderbird DE only problem!!! Thunderbird DE provides two locales (registered in installed-chrome.txt): - first de-DE - second en-US Thunderbird is running in de-DE. If you're installing an en-US extension (it must be the first installed extension), Thunderbird is completely switching to en-US. The same behaviour, if the installed extension provides in install.rdf first en-US and second de-DE. In the other case (first de-DE and seconde en-US) there is no problem! Maybe you're thinking: Okay, Thunderbird DE should only provide ONE locale (de-DE), so it couldn't switch back to en-US. But in this case you'll get chrome-errors, when installing an en-US extension in Thunderbird de-DE. Reproducible: Always Steps to Reproduce: 1. Install Thunderbird DE: http://downloads.mozdev.org/gtl/thunderbird/0.8/thunderbird-0.8-win32-deDE.zip 2. Install an en-US extension or an extension which provides en-US before de-DE in install.rdf 3. Restart Thunderbird. Actual Results: Thunderbird switchs back to en-US Expected Results: Thunderbird should be still de-DE; only the extension should be en-US. In case of an extension providing en-US before de-DE in install.rdf, Thunderbird AND the extension should be de-DE. Related Bugs: http://bugzilla.mozilla.org/show_bug.cgi?id=258878 http://bugzilla.mozilla.org/show_bug.cgi?id=226827 http://bugzilla.mozilla.org/show_bug.cgi?id=258725

The Japanese language package was introduced into Firefox and this problem reappeared. It is because English came to be registered into the following files by No. 1 as a priority of language. Since this entry was not registered, the problem did not have Firefox 0.9.x. File: ~/Library/Application Support/Firefox/default.xxx/chrome/chrome.rdf The newly added entry: <RDF:Seq RDF:about="urn:mozilla:locale:root"> <RDF:li RDF:resource="urn:mozilla:locale:en-US"/> <RDF:li RDF:resource="urn:mozilla:locale:ja-JP"/> </RDF:Seq> Mac OS X 10.3.5 Firefox 0.10PR

Fixed with bug 258725