Editing function locking up in Firefox 18.0.1

Hello,
Recently starting having this problem:
If I am in a page and highlight something, or try to paste, or try to type in text, I get that "boop" sound and nothing happens - something is preventing the action. It happens when I click to a different location in the page and then try the action. To cure it, I have to click out of the page onto something else and go back in. The first time I'm in, I can complete the action. But if I then click to another part of the page - say, to highlight a phrase - the problem will happen. Driving me buggy...
It is not happening in Chrome.
P.S. I am not very tech-savvy.
Thanks!