In order to change the default reader for PDF files (to open PDF files with Firefox's internal PDF reader), follow these steps:

1. Click the menu button and click Options (Preferences for Linux and Mac).
2. In the Options (Preferences for Linux and Mac) tab (window for previous than 38.0 firefox version), select the Applications panel.
3. In the Search field, type PDF. You should find Portable Document Format (PDF).
4. On the right hand side you should find an Action column. Use that to select your favorite PDF reader. In order to view PDF files in Firefox, choose Preview in Firefox.

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Thank you.

Hi Wesley, thanks for the reply. But that doesn't address the issue. Firstly it has been set to "always ask" which I prefer, because I want to save some PDF's. But most of the time I wanna do a quick scan through and open one in Firefox.

Up until this update it just did what I told it to, and this isn't just with PDF's, any file type I choose to open with another program it still just saves it regardless of the save or open choice I make. So what's the point then? Been using Firefox for years, a few weeks ago it was simple. Choose open: a tab appears, choose save: file gets saved. Did Firefox take away this option or what?

Is the PDF file actually saving in your downloads folder or is it saving in your system's temporary folder?

Whenever you open a file instead of saving it, the file will still be downloaded and saved on your system in order for the program to access to it. The file will also appear in your download history. This is the normal Firefox behavior.