How many extensions and plugins are you running? Mine loads up @ 38mb, here's what I load up with Firefox:
Extensions
AdBlock Plus 1.1.2
Adobe Download Manager
Download Helper 4.6.5
Ebay Sidebar for Firefox 2.0.3
UnPlug for YouTube 2.024
Plugins
Adobe Acrobat 9.1.0.163
GetPlusPlus for Adobe 1.6.2.49
iTunes Application Detector 1.0.1.1
Java Deployment Toolkit 6.0.170.4
Java Platform SE6
Mozilla Default Plugin 1.0.0.15
QuickTime Plugin 7.6.5
Shockwave Flash 10.0.32.18
Silverlight Plugin 3.0.4
Windows Live Photo Gallery 14.0.8081
Extensions
BetterPrivacy (super cookie safeguard)
BugMeNot
CustomizeGoogle
FEBE (extensions backup)
Fetch Text URL
FireShot (screencap program)
Forecastfox
IE Tab
Interclue (shows links in pop-up window)
Keyscrambler
MapThis
MediaPlayerConnectivity
NoSquint
Reminderfox
Screengrab
TabMixPlus
Zoom toolbar
Plugins
DivX Web Player
Java Platform
DRM
QuickTime
RealPlayer
Flash
Silverlight
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I usually run video links in a separate browser. I like QTWeb Browser for that. I open it each time I need to view video.
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Firefox has consumed another 5,000K in the last 10 minutes, and I have only one tab open.
Even if I close all tabs, the memory usage does not shrink. That is Firefox’s problem — it won’t release memory when web pages are closed.