Mozilla updates Firefox, ESR and Thunderbird: checking each branch avoids maintenance errors
An official advisory dated 28 October brings together the new Firefox and Thunderbird version references. The standard and ESR branches have different versions: they must not be managed as a single update.
News summary
Advisory AV26-1100 from the Canadian Centre for Cyber Security, dated 28 October, lists Mozilla updates associated with 27 October. It identifies as affected Firefox versions earlier than 159, Firefox ESR versions earlier than 115.44 and 153.6 depending on the branch, and Thunderbird versions earlier than 159 and 153.6 depending on the branch.
This distinction matters for businesses that maintain an ESR version for compatibility or use Thunderbird for email. An organisation may have several branches installed on different devices. The advisory recommends consulting the related bulletins and applying the necessary updates; on its own, it does not provide sufficient grounds to claim active exploitation.
Source: Canadian Centre for Cyber Security — 28 October 2026
What to check in your business
Determine which branch each device uses before setting the target version. Do not force a migration from ESR to the standard branch by confusing the bulletin's references.
Check that the browser and email client have finished updating and have been restarted. Include secondary installations and devices that do not connect regularly.
At Seintec, we help you organise maintenance for your devices and servers, check versions and plan changes. Contact our team.
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