Reproduced in RC3. I just used the stub installer for the first time, and it all went well until I came to the point where the downloaded started. It stalled for a very long time, and said "0k/sec, ... 0k downloaded" and then suddenly after about 1 minute, it jumped to 24% or so, and started downloading as usual. Looks like the download started right away, but the UI didn't update until after a very long while. This is a serious issue, IMHO, since I was about to close the stub installer altogether (I thought it didn't work).
I think this could be a dupe of bug 145776. ftp.mozilla.org seems to have to network problems. Please try a newer build and see if you can reproduce. I cant..:)