"It's Flash. Mozilla themselves even pinpoint flash as one of the major extensions notorious for high memory usage.
Or you can block/control specific instances of memory usage, such as for Flash Player, via extensions or configuration options. Flash images in particular are pervasive in modern web pages (as ads, videos, etc) and so are a common source of memory usage.
Even Mozilla themselves recommend FlashBlock which can be found here.
If you are using an older version of Flash, it is recommended to upgrade to Flash Player 9 as soon as possible:
Also, version 220.127.116.11 of Flash plugin can continually consume memory while you are on a web page that includes Flash content.  Workarounds are to remove the Flash plugin, block Flash content, or manually navigate away from pages with Flash.
Celeron is also an older chip and Flash can require a fair bit of CPU power at times."