On Ubuntu 15.04 (vivid), which is in beta, and a recent install of Firefox nightly 40.0a1 (2015-04-15) with no plugins enabled (including the ubuntu plugins). In about:config set browser.ctrlTab.previews to true. Open more than two tabs so that the tab preview window will open during navigation. Press ctrl+tab. The tab preview opens, further presses of ctrl+tab cycle through tabs, but releasing the keys does not close the preview window and change tabs. I can press escape to close the window without switching tabs, and I can click a preview to change tabs, but I can not change tabs just by pressing and releasing ctrl+tab. I also saw this behavior in the build of Firefox included in Ubuntu.
I filed a slightly better bug with Ubuntu, which may be the more appropriate place: https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1444788
This looks like it is fixed, I can see layout issues in the latest Ubuntu however I can't replicate this experience you have. Please let me know if this is still is happening.