It sounds like one of your CTRL keys on your keyboard maybe stuck down. Have you got "Sticky Keys" enabled, it's a feature in Control Panel, in the Accessibility Options. It sticks keys down like the CTRL, ALT and Shift keys for people who can't press more than one key at a time, but sometimes can be activated by accident.
Or it could be that you keyboard is malfunctioning and has the CTRL key stuck down, try pressing P or S in Firefox whilst pressing no other keys. If when pressing just P on it's own you get the print dialog box, or the Save dialog box when pressing S then your CTRL key is stuck down.
Normally if you hold down CTRL and click a link it will always open in a new tab, so this is why I think your browser maybe reading a Ctrl key press as well as a click.
Also is your mouse one of these programmable types (most cheap mice aren't, but some top of the range mice the buttons can be reprogrammed with the installation software). If your mouse has configuration software check in that to make sure the left click hasn't been altered to Ctrl+Click. If it's just a cheap standard mouse using just standard windows drivers then you probably won't have this option.