As I open more windows or tabs in Firefox it becomes slower and slower, finally unusable. It can take as long as 20 seconds to scroll the screen, which tends to scroll in large jumps, even when smooth scroll is enabled. The problem is worse when smooth scrolling is enabled, but is bad enough even when it is not. Other applications seem to have scrolling slowed somewhat when Firefox has many windows open. The problem seems to be related to flash, since when I kill xulrunner/flash, graphics performance gets better, but still does not always become acceptable. Swapping between Firefox windows and tabs is normal.

It looks to me like a memory swapping issue, but I have no evidence beyond my gut.

Right now Firefox has 30 tabs open on all its windows. Performance is bad, though not intolerable.

The system is:

4 processor x86_64 system
Fedora-19 with all updates installed,
Firefox-25.0,
Nvidia GeForce 8400GS with the proprietary driver.

Any ideas what's going on, or how to investigate?