Can't set Foxit Reader as standard PDF reader in Windows 7

I started to use Windows 7 and wanted the Foxit Reader again as my standard PDF reader. However, even if I check the box 'yes, pleeeease open pdf aaaalways with this program' I have to define again for every subsequent pdf which program I want to use (and I only have Foxit Reader). Moreover, that might also be the cause for the unability of my Firefox browser to open Pdfs at all? I already searched for the menu where I could tell Firefox how to open a pdf, but I can't find any button at all.

Would be great, if somebody could help me, since I need pdfs every day...!! Thank you.

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