This appears to be the account manager.

When I installed the nightly build and started it the first time, it converted my Netscape account stuff as it always does.

The next time it started it came up to account manager and showed account aax, but said it couldn't find it to use it, so I made a new account, ajg and copied all my bookmarks and such from my older Mozilla. This woked just fine, except account manager came up on each start forcing me to select the account, which I don't need, so I used account manager to delete the unused aax account.

On restart - lockup at flash screen.

I restarted the computer to clear mozilla out (nothing could kill it) and reran the unzip to reinstall. Now, no problem. It uses the ajg account as default.

I am pretty sure I also had the lockup problem some time back when I prevented it from converting Netscape, so my suggestion is to make sure you have a fully operational Netscape 4.61 on the same drive as the mozilla install and let it convert the accont info. Hopefully it will then run OK.