Strange problem with web browsing (MBA13, Yosemite)

Hi there!

I got a MacBook Air13 (still on Yosemite 10.10.5) since almost a year and it was just fine until recently it started doing the strangest things with my Internet.

First it started having issues with speed but that was actually because of the crappy router. But now it will only open certain pages (and that decently quick actually) like Facebook, YouTube, Apple and all Google services (and strangely enough my University Library page, so I crossed out the conspiracy theory) but it just won't go any further. Regardless if I use Firefox or Safari.

I tried messing with DNS and locations, clearing cookies, freeing space and all the usual stuff. Also checked the Internet connection but it's fine. Neither can I think of anything that I did different than usual...

Maybe someone has a clue what's wrong?

D.