Oddly enough, things started out better this morning... I browsed a bunch of topics and only got one pop-up (and five notifications that pop-ups had been blocked).
So, just now, I came back to the site to report that things were getting better... and I got five or six popups just while trying to navigate from the front page of the site to this thread (along with several "pop-up was blocked" notifications).
Yes, I have the Firefox pop-up blocker enabled, and I have Firefox configured to open new windows in a new tab instead. I've had it set that way ever since they introduced tabbed browsing years ago. I'll try un-checking that option and see if that makes a difference, but I hope it's not that because I greatly prefer new tabs over new windows, especially when using my laptop.
I'm also going to delete my railroad.net cookies and see if that makes any difference. I'm not sure why it would, but I'll try it anyway. I don't have any hair left to rip out.
Pan Am Railways — Boston & Maine/Maine Central — Delaware & Hudson
Central Maine & Quebec/Montreal, Maine & Atlantic/Bangor & Aroostook
Providence & Worcester — New England — GE Locomotives