Idling Firefox is writing to the hard drive

I wonder what.  With Antique Laptop powered up, XP running, Firefox 20.0.1 active, and laptop just sitting there, no one running it, I see continuous I/O writes in Task Manager, and I see the hard disk LED flickering all the time.
    Translation: when it has nothing to do, Firefox is stashing something to the hard drive.  I wonder what it might be.  Cookies?  That many cookies?  That's hard to believe.  A virus?  Has my Firefox been infected by something awful?  Should I wipe it off the hard drive and down load a fresh copy?  I ran Spybot, Malicious Software Removal, and Malware bytes just a few days ago.  No hits. 

Followup.  Problem seems to be an obsolete version of Adode "reader" the Firefox plugin, not to be confused with the Adode PDF reader.  Anyhow, run Firefox.  Click on "tools" on the Firefox menu bar.  Select "Add -ons"  This opens a window with four selections.  You want the one named "Plugins".  "Plugins are bits of computer code written be third parties that load into Firefox and can do nearly anything.  A defective plugin can do all kinds of evil things.  At the top of the list of plugins there is a highlighted text line reading "Check to see if your plugins are up-to-date".   Click on it and after a little thrashing it will tell you what plugins need updating. 
   In my case a plugin named "Adobe Reader", a Firefox plugin, not to be confused with an application of the very same name used to view .pdf files, needed update.  It's slow but it works.  The strange disc accesses went away after update finished.  Update is SLOW, but it does work. 
   The Adobe update is "bundled" with some McAfee anti -virus.  I failed to uncheck the teeny weeny camoflaged box to skip McAfee.  Which made the sluggish download more sluggish.  Just for grins I ran the McAfee antivirus and it locked up.  After waiting half an hour for something to happen, I killed it off with Task Manager and zapped it off disk with Add/Remove programs.