UPDATE May 2020 > +1 star/ -1 star
Even though there is no further development on this extension (because of the "keep it simple" policy), it has survived over many Firefox updates and is still fairly reliable (+1). Unfortunately, when titles are included in hyperlinks (i.e.,
The known workarounds: 1) quickly move the mouse pointer away from the link and hope that the pop-up symbol is still remains on screen just long enough to hover on it or 2) move the mouse away from the link, hover over the link again and hope that the title text pop-up does not completely hide the SLP-pop-up symbol.
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UPDATE Sept. 2018: 4 -> 3 stars
The lack of settings and intermittent quirkiness (see minus-points below) for this great tool is really silly, given how simple it would be (modifying link_preview.css and link_preview.js!)
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Not having a replacement for CoolPreviews ("Preview and share links and media without leaving your current page.") prevented me from updating Firefox since version 45 or so.

This plugin, though still quite rudimentary, provides the essentials not only for viewing links on mouse-over in a popup window (which can be pinned) and still keep the source page in view, but also for working productively (e.g. for web development, for making CMS backend settings) in the popup window.

+ very quick response when mousing-over the preview popup symbol (nice!)
+ minimal essentials (independent forward/backward navigation and scrollbars, keep-open pin, option to open previewed link in new tab)
+ non-obstructive (does not cover other browser elements such as bookmarks, menus, sidebar, scrollbars)

- no popup window resizing or moving (stubbornly covers just over half of the current page) either on-the-fly or in any settings (is there a hack for this?)
- popup symbol sometimes sluggish to appear or does not appear at all on first mouse-over on a link

Looking forward to updating this GREAT, INDISPENSABLE extension to FIVE STARS in the future!