Firefox uses huge amounts of memory - 2.5Gb yesterday with only 6 tabs open.

Firefox (v 7.0.1) slows to a crawl regularly (Running under Windows 7) and when I check memory using Windows Task Manager memory useage can be well in excess of 1Gb and yesterday evening it was almost 2.5Gb, and the Firefox process was using 25% CPU. I rarely have more than 10 tabs open at any one time. Stopping Firefox and re-starting temporarily speeds things up but the memory useage creeps up within about an hour. As I sit and type this Firefox is using between 10 and 15% of CPU and memory use if slowly creeping up by about 1Mb to 1.5Mb per second with occasional decreases of 1 or 2 Mb. Other programs run OK and other PCs on my home network (using older versions of Firefox) do not have this problem.