Fix Custom Font Fallback Overrides in Mozilla Thunderbird — HubSpot Guide for Editorial & Content Newsletters
Editorial newsletters rely on content layout readability and structural text spacing. If fonts default to Times New Roman or layout borders break, subscriber engagement metrics suffer. Below is the targeted integration guide to fix Custom Font Fallback Overrides inside Mozilla Thunderbird templates when using HubSpot.
Technical Breakdown
Mozilla Thunderbird uses the Gecko which lacks support for standard rendering rules. This causes Custom Font Fallback Overrides elements to break or collapse when rendering templates.
HubSpot Behaviors
HubSpot's COS template manager automatically wraps content blocks in dynamic div structures to support their drag-and-drop system. These extra wrappers force styling inheritances that override inline td paddings, leading to spacing collapses in Outlook. Setting explicit table element attributes is crucial. Keep your editorial body clean. Strip redundant CSS class definitions using Vaporize to ensure the newsletter stays under Gmail's 102KB clipping ceiling.
Remediation Code Block
To prevent Outlook's Times New Roman fallback override, wrap your custom font declarations inside an @media block that Outlook ignores, or use MSO conditional statements to override font-families specifically for Microsoft Word rendering engines. In your HubSpot campaign manager, make sure you configure your links to run via {{ unsubscribe_link }}.
<!--[if mso]>
<style type="text/css">
body, table, td { font-family: Arial, sans-serif !important; }
</style>
<![endif]-->
<style>
@import url('https://fonts.googleapis.com/css2?family=Montserrat&display=swap');
</style>Live Code Sandbox Test
How to Deploy inside HubSpot
Copy the fixed VML or table wrapper code block from the remediation box above.
In HubSpot's Design Manager, edit the modules section. Create a 'Rich Text' or 'HTML' module block. Double-click the block, choose 'Tools' or 'Advanced', click 'Source Code', and drop the code snippet inside.