Hi,
Here's my personal experience with high memory usage when loading Firefox..
The loading of Firefox was really slow, and I saw that the memory usage went up to 200.000 Ko when loading, before going down to ~24.000 Ko. Consequently, other process have to go in the swap (I have 512 Mo of RAM), and it was really slow.
When loading Firefox in safe mode, I didn't saw this phenomem. The problem came from the ForecastFox extension (version 0.9.3).. But in the following page, "Problematic extension" :
http://kb.mozillazine.org/Problematic_extensions, you can read for ForecastFox : "Prior to 0.9.2 - Memory leak - Upgrade to version 0.9.2 or later". Apparently, the problem is not fixed yet !
So, I've uninstalled ForecastFox and all is perfect again !