User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:25.0) Gecko/20100101 Firefox/25.0 (Beta/Release) Build ID: 20131025151332 Steps to reproduce: Opened a new Firefox window with the program previously closed. Bar will not appear if there's already a Firefox window open. Actual results: A dark gray bar appeared at the bottom of the window, above the status bar, which disappeared after a short period of time. Expected results: No objects appear and disappear on the window.
Please check if the issue occurs using Firefox in safe mode (with your addons disabled): http://support.mozilla.com/kb/Safe+Mode Or on a new, empty profile: http://support.mozilla.org/en-US/kb/Managing-profiles#w_starting-the-profile-manager
With a new, empty profile, the bar still appears. The bar disappears when clicking the "Do not make Firefox my default browser." However, with safe mode on, the bar does not appear.
Does it depend on whether hardware acceleration is enabled (Tools → Options → Advanced → General)?
Yes. After opening and closing Firefox multiple times, every time hardware acceleration was enabled the gray bar appeared. Every time hardware acceleration was disabled the gray bar did not appear.