Firefox Smart Window beta now runs on Mistral Small 4

Mozilla and Mistral announced a partnership on September 16, 2026 that puts a Mistral model behind Firefox's AI browsing assistant. Mozilla's post names the model as Mistral Small 4 and says the Smart Window beta runs on it in the United States and Canada. The detail that matters for anyone deciding whether to allow this in a browser they manage is that the model is not locked in.

Mozilla says users can choose from other AI models in every market where the beta is available. The model picker shown in Mozilla's own announcement image makes the shape of that choice concrete. It offers "Personal: Mistral Small 4," marked Recommended, above two alternatives: "Fast: Gemini 3.1 Flash Lite" and "Flexible: Qwen 3 235B."

So Mistral is the recommended default rather than the only option. The company frames the deal as expanding competition rather than picking a winner.

What Smart Window actually does

Both companies describe Smart Window as an assistant for work already happening in the browser. It helps make sense of a complex search trail, find something the user clicked away from, and pull together information from the tabs that are open.

That last part is the one to read carefully. An assistant that sources information from open tabs is reading page content, which is a different privacy question from a search box that only sees what is typed into it.

Where it runs, and on what terms

The two announcements describe the rollout slightly differently, and neither gives a Firefox version number.

Item	What the companies say
Model	Mistral Small 4, per Mozilla; Mistral's own post names no model
Status	Beta
Markets now	US and Canada on Mistral Small 4; France with French-language support
Planned	More of Europe later in 2026; Mistral names the UK and Germany
Model choice	Users can pick other models in every beta market, per Mozilla

On data, the more specific commitment comes from Mistral's announcement, which states that conversations "aren't saved on Mozilla's servers by default, and partners like Mistral agree to zero data retention." Mozilla's own post stresses privacy and user control as goals but does not spell out retention terms.

Mistral also says regional language and dialect support is built in rather than added on top of an English-first product. It is the argument the company has been making since it raised 3 billion euros at a 21 billion euro valuation earlier this month.

What this means for developers

There is no developer surface here yet. Neither announcement names an API, an extension point, a WebExtensions change, or any documentation for third parties. If you were hoping to call Smart Window from an add-on, there is nothing published to build against, so treat this as a product change rather than a platform one.

The part that does reach your work is page reading. An assistant that summarizes across open tabs will encounter your pages, including content behind a login, in a session the user has already authenticated. That is worth thinking through if your site shows data a summarizer should not carry into a different context.

For anyone who manages Firefox on other people's machines, the two facts to check are the retention wording and the model picker. Mistral's zero-retention commitment is a statement about one partner, and the model list is user-changeable, so the guarantee a user ends up with depends on the model they select. An enterprise policy that assumes one vendor's terms will not hold if the picker is left open.

Watch for the version number and the regional gaps. The beta covers the US, Canada and France, with the rest of Europe promised later in 2026. That means users in Berlin or London cannot try the feature yet, and no Firefox release note names a version. Both are reasons to verify behavior yourself rather than plan around a launch date.