a possible work around found but need more test. not ideal as setting amt of storage per site for yahoo interactive charts:

open windows control panel, system and security, flash player
in the first tap (storage),
  select ask me all site from storing info...
  click storage setting by Site
  ensure the desired sites that you deem safe, has allow setting

for yahoo interactive charts, you need to allow

 l.yimg.com
 s.yimg.com
 s.ytimg.com
 v4l.yimg.com

so you don't get prompts.

Ideally adobe should refine the setting to allow amount for storage - just in case the site gets hacked or site operator do something unusual

That is a Flash setting and doesn't have anything to do with Firefox, so you would have to check the Flash docs about LSO.

Flash Website Storage Settings panel:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html

Global Storage Settings Panel:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager03.html

Flash Local storage settings Help:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/help02.html

you're right about the flash local storage problem I just offered a work around. however, I'm afraid I failed to find the Website Storage Settings panel under the windows 8.1 control panel

You can open the Flash settings pages via the Macromedia (Adobe) website links.

• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html

You can access more setting pages via the buttons of the Flash object or via the left sidebar.

As an aside, this is not related to the protected mode feature. You can raise or lower storage limits with protected mode either enabled or disabled.