Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

You should update your outdated Shockwave Flash 10.3 r181 plugin.

• https://www.mozilla.org/plugincheck/

You can find the latest Flash player versions for Firefox on this page:

• http://www.adobe.com/special/products/flashplayer/fp_distribution3.html

Do you mean that it shows a second entry on the Windows 7 Taskbar, as though there were two physical windows, even though there really only is one Firefox window?

For that problem, there is a setting you can change in the Options dialog:

"3-bar" menu button (or Tools menu) > Options > Tabs

Uncheck the box for "Show tab previews in the Windows taskbar"