Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Thanks. But the problem is still there......

Okay, so the problem may not be Firefox or its add-ons.

Is it just the bookmarks are blinking? How fast?

Start your Computer in safe mode. Then start Firefox. Try Safe websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

The bookmarks are blinking fast. But only when I'm looking at a video. No problem with the bookmarks in other windows. And ideas?

Is this about bookmarks on the Bookmarks Toolbar?

Are you using Firefox in Full Screen mode (toggle: Command+Shift+F)?

When I enter full screen mode the top and bottom of the video image flash only when the video progress bar is on. When it disappears, the flashing stops. The same thing happens when I'm not in full screen mode too. The flashing only happens when I toggle over the progress bar at the bottom of the video image. When the bar disappears, the flashing stops. ??