Why is Firefox slow?

Why is Firefox slow?
 

I've been using Firefox for a couple of years and I love it. But lately it's been getting really slow and takes a long time to open for some reason and I can't figure it out.

Has anyone else had this problem before?

I had that problem once and I had to delete a file in the mozilla directory in program files, I can't remember what file it was but it ended in rdf.