User Agent Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:58.0) Gecko/20100101 Firefox/58.0 Build ID 20171023100252 [Affected platforms]: Mac OS X 10.12 and Mac OS X 10.13 [Affected versions]: Nightly 58.0a1, Firefox Beta 57.0b10 [Steps to Reproduce]: 1. Launch Nightly 2. Resize the window 3. Maximize the window 4. Fullscreen the window and exit from full screen. [Expected result]: The tab will return to the tabbar without any problems [Actual result]: The tab jumps when exiting from fullscreen

Thanks for reporting! I'm able to reproduce this. I noticed that if we didn't call TabsInTitlebar.updateAppearance(true) in FullScreen.toggle(), the tabs would remain in their incorrect position after exiting fullscreen. We seem to fail to restore window.STATE_MAXIMIZED properly when exiting fullscreen.

This was fixed by https://hg.mozilla.org/integration/mozilla-inbound/rev/e354a018b703 from bug 1356920. \o/