The two main issues that Safe Mode typically tackles are: (1) disabling all Extensions, and (2) disabling hardware accelerated graphics. You can try these manually normal mode to see whether either makes a difference alone.

You usually need to restart Firefox after making these changes before they take effect.

To disable extensions:

orange Firefox button or classic Tools menu > Add-ons > Extensions category

To disable hardware acceleration:

You usually need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.).

orange Firefox button or classic Tools menu > Options > Advanced

On the "General" mini-tab, uncheck the box for "Use hardware acceleration when available"

If you restart Firefox, is there any improvement?

You can also try to boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps.

• http://windows.microsoft.com/en-US/windows7/Advanced-startup-options-including-safe-mode

Thanks for the reply. I ran FF in safe mode and disabled the hardware accl. There are no problems copying in FF in safe mode (I tried this one week ago). I booted back up in regular mode and the problem seems to have stopped. Though last time I ran in safe mode and started up again it also subsided, but then after a few days the copy problem came back. Hopefully it lasts this time. If it comes back can I open up this post again?

The thread stays open for a couple of months unless a moderator closes it sooner (usually if it gets too long and way off topic, which doesn't seem to be the case here).