It works on my macOS install, so I would investigate if any keyboard shortcuts have been tempered on the system preferences.
A way to check that if to try to set the tab cycling on another shortcut on firefox. If it works you can be quite confident that something else is at play.
The fact that you have very few results in your issue research is also a strong indicator that the problem might be specific to your setup.
yeah. That was it. ctrl+tab was set to cycle through app windows system wide. I had been grumbling for almost half a year. I've set it to ctrl+alt+tab. I'm looking forward to discover the unexpected incompatibilities.
I had set ctrl+tab to "move focus to the next window" as cmd+tab shows applications. It was cycling through tabs anyway on Safari and Chromium
If that helps, i've set app windows cycling to ⌘+<key above tab>, which is ⌘+@ on my azerty keyboard. I don't think it's a commonly used shortcut on qwerty either.
A way to check that if to try to set the tab cycling on another shortcut on firefox. If it works you can be quite confident that something else is at play.
The fact that you have very few results in your issue research is also a strong indicator that the problem might be specific to your setup.