Firefox only (Win/Mac/Linux): Firefox extension InstaClick makes up for your lack of a middle mouse button by re-mapping the right-click function to open links in a new tab.
Once the extension is installed, using right-click on any link will open it in a new tab—if you want to see the normal context menu for a link, simply hold down the Shift, Ctrl, or Alt keys while right-clicking on the link. There’s no UI, preferences or anything else—it just works exactly as advertised, and since it only applies when clicking a link, it’s not very intrusive.
InstaClick is a free download, works wherever Firefox does. It’s one of those add-ons that isn’t right for everybody, but if you are stuck with a laptop that doesn’t have a middle-click button, it’s well worth a look. Thanks, emellaich!
InstaClick [Mozilla Add-ons]