I'm using IE. My parents had Safari and I hated that damn thing, and I've never tried Firefox.
I can try checking the cookies again, see what the deal is there, 'cause I did have some trouble with Yahoo lately, not so much signing in, but with reading the news and it crashing IE on me.
Could be 'cause it's old, too, I've got everything on this ancient computer of mine as updated as I can get it. If that's the case, meh, I'll deal with it.
It worked so long as the browser was open. If I quit or it crashed, and I came back, then I'd have to sign in again.
But it looks like it's ok now, 'cause I actually did a double take this morning, realizing that I didn't have to sign in, so it must be fixed.
I also have a nice anti-virus program that I use for virus check, defragging, and such, but far as I know, it doesn't turn off cookies.
It appears to be fine now, so, if it happens again, I'll check with IE, look at my cookies and stuff, see if that's a problem I didn't know about.
Otherwise, if it's not working, it might just be that my stuff's getting old and I can't really do much about that short of getting a new comp.

Thx for the help guys. At least I wasn't the only one with this problem for a little while.
