Print Hint, a Firefox extension developed by Tony Murray notifies on the availability of a printable version of the current web page. And it does it smartly as it not only detects the availability of an alternative style sheet for printing but also if there is some link in the page that seems to point to a printable version.
Alternative style sheets are notified in the status bar by default but a more useful way is having the alert (a green go icon) shown directly on the Print button (if you have added it to the toolbar). If no style sheet is available but a link to a printable version is found, an orange arrow is shown. You can then press the print button to get the alternative format.
In my tests it failed to notify some style sheets in the status bar while the Print button was correctly displaying the green icon. Identification of printable versions works with English web pages only as it looks for common terms like ‘Printable version’ or ‘Print this’, but this should get solved as more localizations kick in.
Finally I would prefer the notification to happen inside the location bar. Since it is just another version of the same document it falls in the same category as web feeds which are notified there.
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