Make sure you are not blocking content.

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?

No, the problem disappeared. What is wrong, how do I determine the right Addon which causes this?

BTW I have the ChromeUser.css hack to obtaine multiple row tabs.

Enable and disable the chromeUser.css make firefox run without problems! Let's see the future behaviour.

NO. It keeps cycling after a while with userChrome.css enabled.

It could be the work of one of your add-ons, or it could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All Of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.

My issue depends on the file userChrome.css. After removing it FF runs fine. Also after changing css to css2 it runs OK, but with .css after a WHILE it causes keeping downloading something. Max

The following is a picture of keeping downloading from my left bottom corner in FF, when the tab displayes the moving dot.