On Saturday I visited a friend’s house to see if I could help them with slowness problems on their home computer. This was a six-year-old machine running XP with 448MB RAM. I observed that on startup Windows Update would run and while running, pretty much all the RAM in the system was consumed by Windows, wuauclnt.exe and svchost.exe (which assists Windows Update). During this time, starting Firefox or IE took minutes; the machine would thrash itself senseless. This state lasted for quite a long time, about half an hour, probably exacerbated by my attempts to get stuff done. Once it subsided, Firefox started quickly and ran well.
This is apparently a known problem and some kind of Microsoft regression.
Under these conditions, Firefox startup time and other metrics are bound to be awful.
Update I forgot to mention, but the Microsoft malware checker was also running at the same time as Windows Update and contributing significantly to resource usage. I guess it checks the downloaded and installed updates for malware...