hello, other users had problems using yahoo mail recently when they had addons or external software present which blocked tracking sites (like ghostery, avg or others).

can you check in firefox > addons > extensions if there's ghostery listed there for example? if so, add an exception for yahoo mail: click on the ghostery icon while you're in yahoo's webmail, choose "edit blocking options" & then don't block on this domain...

I checked but ghostery is not listed in extensions. Very few things there, but several different versions of Java, all disabled but one. Any other suggestions?

does it work when you launch firefox in safe mode once?

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems