Saving images with Firefox

Anyone know why sometimes when I open  a pic to full size, and "right-click --> save as..." it gets saved almost instantly, but other times Firefox re-downloads the image instead of using the cached one?

1>>
Ok, I've refined my question:

How does Firefox determine whether or not to cache an image?

Why the fuck would you be bothered if it downloads another image??? You on shitty dial-up???

>>3
>You on shitty dial-up motherfucka???
I fukken lol'd.

>>3
I've done it a few times from an image randomizer, it's like no, I want to save that image, but it goes and downloads a different one to save.

>>1

OP here.

In case anyone wants to know --  After Google'n my fingers raw I came across a comment somewhere that said if Firefoxs' cache gets corrupted IT WON'T EVEN BE USED.  They suggested closing Firefox, and deleting the cache files.  For my machine they were here:
C:\Documents and Settings\<MY ACCOUNT NAME>\Local Settings\Application Data\Mozilla\Firefox\Profiles\<RANDOM CHARS>.default\Cache

I followed the advice, and now images are caching again :)