The newer versions of FF have been called "resource hogs". Log on with Explorer and compare the length of time delays.

Their problem forum discusses this at times. It will also take longer to close.

I have a slow open too. This is because of our free Avast and also the "looking for updates" feature. I've grown used to this now, but it is aggravating. If you try to click on things and "rush" it, you can freeze your computer.

When you sign on don't rush your computer at all.

One thing that really slowed us down, was the new Yahoo bar.
That can take a while to load some days too, depending on your DSL traffic. But I like the Yahoo bar, but I just tolerate it.
Our back up laptop doesn't have the Yahoo bar and it loads much faster!

There are ways to speed up loading...but that involves removing things from your desktop tray. I always get nervous about that... so we haven't tried it. But if you Google "slow start up fixes for Firefox" or something like that you will find articles about it.