I've had exactly the same problem in Firefox, also for about a month now, and it's not limited to Pprune; same happens with a few other sites. Press the back button and you get a blank screen and "done" in the lower left-hand corner. "Reload" gets you back to where you wanted to go but it's a pain.
I'm not usually conspiracy minded but the problem began around the time I downloaded the latest version of Google Chrome to see how that stacked up against Firefox.