Why does Firefox comsume my computer when in Yahoo Mail

Question

For the past week, something is effecting Firefox when in Yahoo Mail. It does not happen at any other web site.
# I do not allow automatic updates of any kind. No changes or additions (unless sneaky stealth attacks?) had been made to Firefox since update to version 12 in May. The only add-on I have is the Mozilla Archive Format, which I've been using for years.
# Windows and various security applications are up-to-date as of very recently.
# Several up-to-date anti-malware programs have found nothing on my computer.
# I tried Chrome and Opera. They do not have this problem, but I don't like either of them.
# I uninstalled Firefox, deleted everything related I could find, downloaded the current version (13) from Mozilla, installed, and rebuilt all my preferences. I got my bookmarks back by restoring from a bookmark backup. This changed nothing.
# I tried "Restart With Add-ons Disabled" in Firefox, but that did not help.
Firefox starts doing something very intensive once in Yahoo Mail. Sometimes it is in this state when it displays the first screen of mail boxes after I sign on. More often I can do a little bit of something, such as open a mail box or a single e-mail therein before the problem starts. From my perspective it starts this aberrant behavior just when I click on the next thing: open another e-mail; open a different mail box; delete an e-mail.
Normally while in Yahoo Mail, or at any other web site, Task Manager reports CUP usage at from 0% to 5%, occasionally a few percent more for a very brief flash. When this Yahoo Mail thing starts
# The CPU usage jumps to 25%, with occasional bursts up to 30+%. Task Manger shows it is due to the process firefox.exe*32.
# The browser will not respond to any controls. I can do nothing with the keyboard or mouse. No menu will open. This has never ended by itself.
# Task Manager reports the immediate drop of network activity to zero -- nothing going on over the phone line -- or so Windows thinks. Disconnecting the phone line does not release the browser or lower its CPU use.
# A few times the entire computer has locked. I have only been able to get out of it with the hardware reset button.
# Sometimes Firefox is totally unresponsive itself but I can cancel it via Task Manager.
# Sometimes the one and only thing in Firefox that responds is the "X" in the upper right hand corner. The browser screen disappears immediately.
# BUT, Task Manager shows the process firefox.exe*32 still running at 25% CPU. I can end said process with Task Manager, whereby CPU usage immediately drops to 0%, or somewhere close.
# Whatever is going on through Firefox prevents me from doing what I want -- and it seems probable that IT is doing something I don't want done, although I have not yet seen symptoms of that.
Where can I go from here?

Answer 1

Hi,
You can consider the [https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems Reset Firefox] feature via '''Help''' ('''Alt''' + '''H''') > '''Troubleshooting Information'''. If it is okay, you can then install the Mozilla Archive Format add-on.
If you have security software installed you can try to delete all existing entries for Firefox and its sub processes in the different configuration modules and try to fully trust/allow Firefox afresh. Even after network access is completed, Firefox would still need unlimited access to its disk cache/memory as well as its files. Please see [https://support.mozilla.org/en-US/kb/Firewalls this].
[http://kb.mozillazine.org/Firewalls AV/Firewall Configuration]