The message bar styles have an outer hbox with an image, description and optional button. This has caused some layout issues, specifically in the new browser language dialog in preferences. Reduce the markup so that it is just the description with a background image and an optional button. <description class="message-bar message-bar--warning"> Oops, something went wrong. <button class="message-bar-button">Try again</button> </description> This should allow localisation like this: <description class="message-bar message-bar--warning" data-l10n-id="error-message"> <button class="message-bar-button" data-l10n-name="button"></button> </description> data-l10n-id = Oops, something went wrong. <button data-l10n-name="button">Try again</button>