After a lot of web surfing, Firefox bogs down and moves slowly, and the only fix is to quit and reboot?

When I've been doing a lot of web surfing - especially on a single site - Firefox slows down and barely more. Toolbar functions and cursor movement also slow. There's a long delay when typing information into online forms - I'm usually several words ahead of what's been entered. It usually strikes suddenly -- first signs of sluggishness usually quickly followed by everything barely moving. Even when I Quit Firefox, there's a very delayed response in all action. But after quitting and rebooting, Firefox opens and is running fine again.