The information you give does not give sufficient clues. Should you have a problem with a virus, you should informed by your program, whatever it is.
And, you don't tell us a thing about your operating system, or the age of your machine, nor the version of Firefox you use (is it the same version which was working fine before?).
My computer is old, and 512 MB memory does not cut it. I, never, saw, a not responding message, but it takes over a minute to be operational. In fact, I, often turn on to see how the task manager is doing, looking at firefox.exe