Firefox slow and getting slower?

I was a Firefox booster a few years ago.  It was a refreshing alternative to IE with some great features.  But something happened.  Firefox became slower.  It took longer to load. I started using IE whenever I was in a hurry. Then performance decreased.  It took longer to open a page. At that point I abandoned Mozilla except for certain websites that struggled with IE.

I wondered if I were just imagining such things, so today, after experiencing long delays with Firefox, I decided to speed test it against Chrome, Safari, and IE .  Not a scientific test, but a test nonetheless opening the same web page from a link.

The results were predictable for these browsers.
Chrome:  Less than 2 seconds
IE: Less than 3 seconds
Safari: Less than 3 seconds
Firefox:  13 seconds

Perhaps it is some plug-in I use.  Or some setting.  I would be interested to know if you see similar results.