Does anyone know why FireFox would start using more memory then it used to?

A few months ago Iwould always have lik 10 - 12 tabs open all day and FF would only use 90,000 - 140,000 memory (in the task manger) but since a few months ago I started closing some of the tabs Ihad open and now I only have 3 - 6 tabs open and FF is using 160,000 - 250,000 memory.

One of the reason Istarted using FF instead of IE was because FF used less memory than IE, but now FF is using just as much.

Ihaven't changed anything since when it was using the less memory to now, when it using a lot more. Istill have only 36 processes like I normally do.

(new questions below)