Unwanted and unusual dialog box when opening download folder

[Moltke]

Hey Moltke,

Whatever you're downloading is something that is attempting to install to your system. Right click the mouse over the folder and select a specific application to open the folder, like leafpad, or archive manager.

TC

Well, it happens with anything I download; videos, documents, anything! I installed pcmanfm and made it the default file manager, I changed it back to thunar but the issue persist. I'll uninstall pcmanfm and see it that works. By the way, I forgot to mention but it all started when I installed cairo dock which I already got rid of because of it. I thought it'd been solved but it's not
added: Nope, didn't work, issue persist. I used synaptics to uninstall stuff, and while I was at it I ran a search for "cairo" and it delivered some results, so I have some cairo stuff in my system. Should I get rid of those?
Added: I just realized that it only happens in Firefox. I just tried in Opera and everything works as it is supposed to. So this might be a Firefox related issue  :o