I have a similar question: is there a way to open PDF files in browse rather than the PDF viewer? An additional option in the context menu would be good, just like "open in browser" or "open in (external) PDF viewer".
I'm not sure if you're referring to Zotero 4 for Firefox or Zotero 5. There's no longer a full Firefox version in Zotero 5, so opening in the browser doesn't really make sense — PDFs simply open in whatever the default viewer is for PDFs on your system. (And in Zotero 4 for Firefox whether PDFs opened in the browser was controlled by Firefox settings.)
I am referring to Zotero 5. My point is that I open every PDF link in Firefox, read it in Firefox, and then save PDF from Firefox to Zotero, so opening PDF in Firefox is natural -- that's why I open PDF in Firefox. The PDF viewer is actually to edit PDF in my case. That's just my workflow, and Zotero 4 for Firefox integrated things well.
You can change your default PDF viewer on your operating system to Firefox. Zotero for Firefox is discontinued because Mozilla no longer supports the extension framework it used, so you can’t really run it from GitHub or anything like that.
Upgrade Storage
I am referring to Zotero 5. My point is that I open every PDF link in Firefox, read it in Firefox, and then save PDF from Firefox to Zotero, so opening PDF in Firefox is natural -- that's why I open PDF in Firefox. The PDF viewer is actually to edit PDF in my case. That's just my workflow, and Zotero 4 for Firefox integrated things well.
I can do it myself as I just found the git repo.