OK; it's probably due to some settings/add-ons within Firefox that are causing a conflict with Yahoo Mail.
Please press Ctrl+D to bookmark this page so that you can easily return to it...
Now, please go to Help (top-left; if you do not see it, tap the ALT key) and select Help --> Restart with Add-Ons Disabled…
Check these boxes:
-Disable all add-ons
-Reset toolbars and controls
-Reset all user preferences
-Restore default search engines
(Do NOT check Reset bookmarks)
Then click Make Changes and Restart, and that should resolve the issue for you.
If you do not see Help --> Restart with Add Ons Disabled, you will need to close ALL Firefox windows (including the one you are currently viewing this page in, if you are using Firefox) then please go to: Start menu > All Programs > Firefox > Firefox (Safe Mode)... and follow the same steps as above.
Let me know how it goes; thank you.