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this post was submitted on 01 Aug 2026
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Yeah, it's happening for me, too. There's a hidden debug menu in the Fennec settings and when I enable that, there's a "Nimbus Experiments" menu where I can turn on the tab groups.
But in the tab group UI, it says "Beta". Not sure, if it also does that on official Firefox Stable.
My best guess is that Mozilla was A-B-testing tab groups on Nightly or Beta, which is why this Nimbus Experiment exist. And now their backend is getting confused by Fennec.
Maybe there's some build flag to permanently enable the tab groups, which Fennec is missing in their own build, and so the backend sees "unsigned build v153" and is really confused why that doesn't have tab groups active yet or something?
Edit: For what it's worth, I found this issue on Fennec: https://gitlab.com/relan/fennecbuild/-/work_items?show=eyJpaWQiOiIxOTUiLCJmdWxsX3BhdGgiOiJyZWxhbi9mZW5uZWNidWlsZCIsImlkIjoxOTU4MTE0NzF9
Ah this explains a lot! Thanks very much