For the past few months I've been having performance issues with FireFox 29: memory usage climbs and climbs and FireFox's response to user input gets worse and worse. FF usually hangs shortly after memory usage gets close to 3 GB.
By selectively disabling and reenabling my extensions, I determined that the problem is the Norton Toolbar (from Norton 360, version 21.3.0.12). With Toolbar disabled, memory usage seldom rises above 600 MB - even with dozens of tabs open all day - and FF stays responsive to mouse and keyboard input. As soon as I re-enable the toolbar, memory usage starts climbing again and eventually FF becomes so unresponsive I have to use Task Manager to terminate the process.
The Norton Vulnerability Protection extension seems to have no bearing on the problem.