I actually finally just switched to Chrome (I'm a long time Firefox user, all the way back to when it was called Firebird), because of this issue:

https://connect.mozilla.org/t5/discussions/microsoft-defende...

Windows Defender flags things in the Firefox cache as a Trojan. It probably isn't, and it could also have something to do with uBlock Origin, but until that is resolved (I'm keeping an eye on the above link) I can't risk it.