It's not threading, it's processes that's coming up. I would be surprised if each tab didn't already have its own thread, because eg. multiple tabs can load at the same time...

And umm, not sure what I'm doing differently but I usually keep Firefox open for weeks at a time and have no issues. Though I cut back a bit on keeping tabs open, I used to keep like 60-80 tabs open all the time but now I usually clean up when I'm done with something (more to reduce cognitive overhead than resource utilization).