I've just tested it on Windows.
It does what it says on the tin in that it pre-loads Firefox so it starts quickly. The downside is that it the browser no longer saves open tabs when it is closed down. Since I often rely on the session saving feature, I've disabled the preloader.
Another annoying thing is that Firefox also seems to have a memory leak, start a new Firefox session, look at the amount of memory Firefox uses, open a load of tabs, see the memory usage increase, no close all the tabs but wtf? The memory usage doesn't completely drop back to what it was before.
Just tried it now:
Loaded Firefox, default Firefox Google homepage it used 23MB.
Opened the following tabs: 2 Stop Microsoft, 2 Wikipedia (main page), 2 Fedora, 1 Gmail, 1 Youtube playing a video, it used 64MB.
Closed all of the tabs, except for the Google and the memory usage only dropped to 40.3MB!
This might not seem to bad but I can easily see all 504MB of RAM being filled slowing the system to a crawl if Firefox is continuously loaded.
