Hello Maria.

You may be having a problem with some Firefox add-on that is hindering your Firefox's normal behavior. Have you tried disabling all add-ons (just to check), to see if Firefox goes back to normal?

Whenever you have a problem with Firefox, whatever it is, you should make sure it's not coming from one of your installed add-ons, be it an extension, a theme or a plugin. To do that easily and cleanly, run Firefox in safe mode and select Disable all add-ons. If the problem disappears, you know it's from an add-on. Disable them all in normal mode, and enable them one at a time until you find the source of the problem. See this article for information about troubleshooting extensions and theme. You can troubleshoot plugins the same way.

If you want support for one of your add-ons, you'll need to contact its author.

I did that and the button still does not appear. When a new website is loading, there are two icons showing that it is loading, one where it always shows and the other where the close button should be.

Are you using the Default theme (Tools > Add-ons > Themes)?

You can also try "Reset all user preferences to Firefox defaults" on the Safe mode start window - See http://kb.mozillazine.org/Resetting_preferences