See:

• [/questions/801315]

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• Bug 641288 – Preferences window opens offscreen without titlebar, leaves white square when closed, can't be moved

It still happens in SafeMode, and I don't know how to do this:

Panos Astithas [:past] 2011-04-10 02:48:18 PDT

I can confirm that setting browser.preferences.instantApply to true fixes the problem.

I searched for 'instantApply' but couldn't find it.

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.
• Preferences that have been modified show as bold (user set).
• Preferences can be reset to the default via the right-click context menu if they are user set
• Preferences can be changed via the right-click context menu: Modify (String or Integer) or Toggle (Boolean)

Thank you cor-el, that was very helpful. Everything works fine now.

You're welcome