CoolPreviews is a neat Firefox plugin that lets you see what a link leads to without opening another tab or window.
After installation, you'll need to spend a couple of minutes configuring it to your tastes.
You can choose how quickly the CoolPreviews icon will pop up - clicking it opens up the preview window. Alternatively, set the preview to appear as you scroll over links.
You can also toggle the pop up to be locked open or not.
Being able to see what's beyond a link can make browsing faster and more efficient.
For reading news sites, it's particularly good, as you can read stories without leaving the home page. Assuming you have a speedy internet connection, you'll be surprised this isn't an automatic feature of browsers.
If tabs made tab-less browsers seem prehistoric, CoolPreviews does that to tabs.
In testing it seems perfectly stable and doesn't slow down our systems at all.
Some people might just find the pop up annoying, but as it can be configured to appear more or less whenever you want, it should only be as intrusive as you allow.