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I find it really annoying, that firefox on my Mac (OS X Lion) automatically downloads a pdf instead of viewing it in a tab. On Windows this works flawless. But why is it not working on the Mac? Is there a way to change this?
What you need is a PDF plugin for Firefox. For instance, this PDF Browser Plugin will work for that. It will allow you to open a PDF in the browser without downloading it.
More information on this Firefox support doc.