You are unable to copy/paste items (text, images...etc) under the Firefox browser.
This problems occurs when:
Your PC is infected with the Pushow adware .
You have installed an extension that prevents the use of the Copy/Paste feature.
You have been infected by the Pushow adware:
Go to C:\Windows\system32.
Look for the following files in system 32
Transfer them to another folder, rename them, restart your PC and then delete it.
Some file extensions are not compatible with your version of Mozilla Firefox and must be removes.
Simply start your Firefox under safe mode to check out if any obsolete extensions are installed and remove them.
Safe mode allows Firefox to start up even if a theme or extension is not working properly. Under this mode Firefox uses its default theme and extension are not loaded.
Close Firefox application and in the run tab type firefox -safe-mode.