firefix is using a lot of memory. i am haveing to close down to get it upto speed again is this a common problem. i am using window 7 with 4 gig memory. thanks

i find that at start everything runs ok. after 30mins it starts to slow down ie takes time to open sites and drop menue takeing to long to open i only have one site open at a time so i cannot be using that much memery. if i restart firfox it run at normal speed. i am runing windows 7 with 4gig memory. thanks