Firefox seemed like it didn’t want to cache any of images on web pages, and I couldn’t figure out why for a very long time. Clearing the cache didn’t help. Firefox is supposed to be smart and clear out unneeded cache items as you browse, but it wasn’t doing the job.
Turns out that Google Gears uses up reported cache space for saving its offline content, but this space doesn’t free up when you clear the cache. In my case, Google Gears was using up over 500MB of space, but my disk cache size was only set to 500MB total. Essentially, I had 0 free space for my cache.
If you type “about:cache” in your Firefox address bar, you’ll be taken to a page showing some information about your currently cached files, including cache size. Type in “about:config” in your address bar to continue to the hidden system settings. browser.cache.disk.capacity is the property you’re looking for, and make sure to set it to a value much higher than what Google Gears is using. You can (usually) find this by checking the amount of files is your “\Users\username\AppData\Local\Mozilla\Firefox\Profiles\xxxxxxxxx.default\Google Gears for Firefox” directory.
I hope this helps someone else!