If you can log in with one browser and not another, then you need to clear the cookies on the browser you can't log in with.
Really. This works. That is the actual problem. I'm not just making this stuff up.
Clear your cookies.
In Firefox: Tools->Clear Private Data->Turn on the Cookies checkbox, hit the clear button.
If it doesn't work, then you did something wrong and your cookies didn't actually get cleared.
Really, I cannot stress this enough. The problem is *known*. It's well understood. And yes, it is in fact fixed if you delete the cookies. I cannot tell you what it is that you're doing wrong, because I'm not sitting there looking over your shoulder. So try it again. Do it a different way maybe. I don't know exactly what to tell you, but I really do know what the problem is and that this is the solution. Beyond that it is up to you.
Now, if you can't login on *any* browser, then that's a different kettle of fish. The Admin SSL plugin, for example, is known to cause that problem. But if it works on one browser and not the other, then it's a cookie issue. There is no doubt about that whatsoever.