The biggest cause for browser slowdowns, memory usage and performance degradation are add-ons and the more you have installed the likelier it is they'll slow down your browser.
That's not really a secret to anyone, but even people that think they're keeping the computer lean and fast forget one simple thing that will have a big impact, disable any add-on you don't use.
Mozilla has an obvious yet effective tip on its Firefox Den blog
, aimed mostly at novice users but equally useful to more advanced ones as well, just off to Tools>Add-ons select the Extensions tab and check out what add-ons you have installed. Disable any of them you're not using, it will only take a minute, but may save you a lot more time in the long run.