Could you check your Add-ons page to see whether you still have Tab Center? It may have gotten turned on as part of a test and then gotten turned on when the test was over.

You can view, enable/disable, and often remove extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. Then scan down the list on the right side. Any system extensions that Firefox installs on its own are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Disabled extensions cluster at the bottom. Do you see Tab Center anywhere?

Hi, thank you for the information, it is the tab centre that is missing, my add on extensions does not have tab centre anymore. I cannot search for it. I have installed tab center redux as a workaround.

Note that the Tab Center extension was part of a Shield study and only offered to some users. You may have received a special extension as part of a Mozilla Shield study.

• https://support.mozilla.org/en-US/kb/shield