Firefox, no doubt, is a browser that gives you a ton of flexibility with its add-ons. Practically any feature that you might find missing or too niche to be integrated as a full feature, can be added with the appropriate extension or script. Pdf.js is a Firefox add-on that makes up for one such missing feature in Firefox; it allows you to open PDF files in the browser as you would in Chrome. Whether the PDF file is being opened from your hard drive or from a link, the extension opens it in the browser, allowing you to forgo the need of having a PDF viewer installed.
Once installed, click on any PDF file link and it will open in the same tab. You can zoom in and zoom out, jump to a particular page, move to the next or previous page and save the file to your system. A thin bar on the right side allows you to preview the pages or the document structure, and jump to any part of the file. Mouse over the bar and a panel will appear with two buttons at the bottom, which will let you choose between the two views.
To save the file to your system, click the Download button and the save file as dialog box will open. You can choose to open the file in your PDF reader or save it. To open a PDF file from your system, click the Browse button and locate the file on your system to open it.
Alternatively, you can drag & drop a PDF file on to a Firefox tab or use the Ctrl+O key combination to open it.