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This can happen if you have disabled the builtin PDF Viewer. You can check this in "Options/Preferences -> General -> Applications" PDF should be set to "Preview in Firefox".

• https://support.mozilla.org/en-US/kb/view-pdf-files-firefox

..I do have a similar issue: Firefox starts opening the file and asks where to store the existing file, option is, it opens an endless amount of that file...

Sounds that you selected Firefox as the application to open PDF files, you should never do this. You can check the action for Portable Document Format (PDF) in "Options/Preferences -> General -> Applications" and make sure it is set to "Preview in Firefox".

• https://support.mozilla.org/en-US/kb/view-pdf-files-firefox

You get tabs opening endlessly if you select Firefox as the application to handle a file when you get an "Open with" dialog.

• https://support.mozilla.org/en-US/kb/Firefox+keeps+opening+many+tabs+or+windows

Thanks for getting back to me!

It seems the issue is solved by changing the settings towards your proposal, the strange thing remains settings never got changed by myself and things always worked, I do have a firefox update in suspicion causing the problem..