hi Jens_Lyn_IV, this is probably caused by the ctr extension. what would normally happen is a doorhanger panel appearing under the menu button, offering to take a tour through the new features of the release...

Thank you. I tried disabling my other extensions, but not CTR; the UI breaks in a weird way if CTR is disabled, and I have to manually restore it after reenabling the extension.

I thought there was no way to suppress the post-update 'what's new' page, but I discovered the browser.startup.homepage_override.mstone entry in about:config after submitting this question and set it to ignore.