I can't guess why your Firefox does that. And rather than enter the swamp I'll just share I use Seamonkey.
I read this thread:
And I did what it said, but for me, my Firefox is still slowing down. Its starting to really frustrate me. I cant get anything done online because it will lag, freeze and sometimes crash! Any idea's on what could be causing this? My PC runs smoothly. As does Google Chrome and IE but my Firefox doesnt!