Is that a software that does that? What exactly is "Print Edit"?

FF extension to edit & print web pages.

Hi idahouser, do you get this problem on all websites? If it only occurs on particular pages, perhaps the author could take a look.

You may notice that on some pages, such as this support site, all extensions that need to inject scripts into the page will not work. Restrictions on using those kinds of extensions on some Mozilla sites is an intentional special case.

Every website I've checked it out on. It's a problem caused by version 66.0 of FF. I just checked a brand new installation on a new computer. FF home page shows "Cannot be used on this page." The release notes for ver 66 say this, "Extensions now store their settings in a Firefox database, rather than individual JSON files, making every site you visit faster." I'm quite sure that change broke Print Edit WE.

As I'm writing this I've been checking some pages. Interestingly it seems to work on Norton page for searchsafe and on Mozilla privacy page, but no other Mozilla page, and also not on the options page of FF.

It's very hit and miss. It seems to have started working on some pages but not others. It's weird. On some sites when I click the icon I get a "page not ready" or the cannot use message, but when I click on it a second time it opens up.

You definitely can't use extensions that need to inject content into pages on built-in pages or Mozilla Add-ons/Support sites. However, they should work on unrestricted sites.

For the intermittent errors, perhaps the page is still loading? In normal usage, you probably won't need to print/edit the page until you've had a few moments to look it over, so hopefully a slow load won't be a problem too often.