User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:37.0) Gecko/20100101 Firefox/37.0 Build ID: 20141218030202 Steps to reproduce: compared the pages https://www.mozilla.org/en-GB/thunderbird/features and https://www.mozilla.org/en-US/thunderbird/features Actual results: the GB version still discusses the migration assistant that was removed about 20 versions ago. Expected results: the en_gb version should be the same as the en_us version.

This is not a en-GB issue, it's true for all locales. https://www.mozilla.org/it/thunderbird/features Thunderbird pages are being moved to Bedrock these very days. I'd go ahead and mark this is as a duplicate of bug 1103396, which covers the porting (and localization) of Thunderbird pages.