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Foxstuck: Firefox browser bug boots legions of users offline

Outfoxed? Not if you read The Reg

In a hard-to-beat demo of the perils of software telemetry, Mozilla accidentally kicked legions of users offline last week by an update to its telemetry servers that triggered an existing bug in Firefox. Internally, Mozilla is calling the bug "foxstuck".

Firefox periodically reports back some fairly innocuous info, including how long your session lasted, how many tabs and windows you had open, what extensions you have and so on. You can see a list by entering about:telemetry in the address bar.

It's all pretty harmless data. What isn't harmless is if your browser goes TITSUP* and stops you from accessing any website just because it can't phone home – especially if other browsers still work fine.

Two different bug reports in Mozilla's bug-tracking system – 1749910 and 1749957 – reveal the process of identifying and fixing the issue, and somewhat unusually, Mozilla publicly acknowledged the problem on Twitter.

Troubleshooting wasn't helped because two different tweaks both bypassed the issue: disabling either telemetry or Firefox's HTTP3 support both worked.

The telemetry server wasn't directly to blame: the problem was isolated to an HTTP3 load-balancer in front of Mozilla's servers. Cloudflare is a major backer of HTTP3, so if it wasn't the Mozilla telemetry servers, something else behind one of their boxes would probably have triggered this bug instead.

Although many people blamed Firefox auto-update for the problem, it wasn't to blame. The issue has existed at least since the last ESR version, Firefox 91, and the problem also affected people running versions 94 and onwards.

Commenters on various forums have called for a fork of Firefox to remove telemetry support, which just demonstrates that they're not Register readers. We pointed to precisely such a tool months ago. Waterfox users were totally unaffected. ®

* Telemetry Inability Totally Silences URL Processing

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