With every program you use, I recommend you review the options you can set from time to time. Here are some of Firefox screens you'll see ...
I don't have enough experience to know how File Types types are specified, but I do know that if you click on Plug-Ins you get a screen like the one below and by un-checking, for example PDF, then PDF files are downloaded and saved locally for viewing (which I perfer) instead of viewing them inside your browser.
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