I don't think it is the addons, but you can be sure by opening the addons page in FF and disabling those two addons.
Could it be the addons?
Run the system for 24 hrs without doing anything else and see how it is then.
For the update that keeps asking you install it:
First I would change the Automatic Update settings:
Click Start, click Run. In the Run dialog box type sysdm.cpl, and then press ENTER.
Click the Automatic Updates tab, and then select:
Notify me but don't automatically download or install them
If you select this option, Windows recognizes when you are online and uses your Internet connection to search for downloads from the Windows Update Web site or from the Microsoft Update Web site. An Automatic Updates icon and message appear in the notification area of your taskbar when new updates are available to download or to install.
NOTE: If you use this option make sure you manually go to the Windows Update site and install any updates as soon as you see the notification.
Check the installed updates in Add/Remove Programs in the Control Panel. If the KB2538242 update is there see if you can uninstall it. If the update isn't there or if you can't uninstall it go on to the next step:
Click here to go to the Microsoft download page for the Microsoft Visual C++ 2005 Service Pack 1 Redistributable Package MFC Security Update.
Click the Download button. (Make sure the Language is correct).
On the next page, click the box beside vcredist_86.EXE then click the Next button. The download file window will open.
Click Save File and save the file to the desktop.
Close the browser and all open windows.
Double click the vcredist_86.EXE file to install the update.
Hopefully this will install the update and get Windows update to recognize it.