Firefox taking 12 GB of RAM

The other day I was just using my computer as normal, no excessive number of tabs open, when all of a sudden 6 or so programs stop responding and the computer slows to a crawl. I open the task manager to see what's happening and at the top of the processes list I see two firefox.exe processes, one at 7 GB of used memory and one at 5 GB of used memory, as well as assorted other processes with between 30 and 400 MB of used memory. I tried refreshing Firefox and only installing one addon back (Reddit Enhancement Suite), and not thirty minutes later it's running slowly and has two pages above 4 GB again. My Firefox is up-to-date, and I've never had problems with RES in the past. Do you know what could be causing this immense RAM usage?