Sorry a bit of a false alarm actually.
What happened was, I downloaded some new themes the other day and one of the files was corrupted and wouldn't work properly, I only found this out once it was loaded and I couldn't exit that particular theme.
A note for Firefox users however, if you have any problems with the browser, you can start Firefox in it's own Safe-Mode (Click on FF in your Start Menu, it's under the normal mode).
I only just learned this this morning.
Anyhow, it's all back to normal here.
Another thing I did learn too, is the fact that there is a rather prolific Support forum at the Mozilla site itself, if you get stuck.