Just googled some examples, perhaps some of them work, but it was not my case.

These ones needed the i flag for case insensitive match:

• NEJM
• ASM

Extracting doi from Pubmed with "citation_doi" as "dc.identifier" was not found:

• Pubmed

I belive Cell stores the content of "dc.identifier" with 'doi:*":

• Cell

Would you be interested in making this a fully fledged firefox addon? Would love to recommend the extension to some peers, but they are not tech savy. I made a fork in git-hub of the project, because thought that was abandoned. Hope you wouldn't mind, just trying to learn some javascript. If you do, I would immediately take it down.

Cheers!

Its not working correctly after updating sci-hub URL
The way is extracting DOI is not woring properly either. Here is a simple fix:

function getDoiReference() {
// more 'universal' query, i flag makes it case insensitive
  const detectedDoiMeta = document.querySelector('meta[name="doi" i], meta[name="dc.identifier" i], meta[name="citation_doi" i]');
// some webpages store the doi as 'doi: 10....', this removes the 'doi: '
  detectedDoiMeta.content = detectedDoiMeta.content.match(/10[\S]*/gi)[0];
  if (detectedDoiMeta) return detectedDoiMeta.content;
  return null;
}