Have you tried unistalling & reinstalling Firefox? Latest version if required click here, though you can update it from within the application.
Safari have had some security issues that they've fixed for the time being by the looks of it click here. I was thinking of trying it today but came across a not-so-glowing independent review from a Mac user himself click here. No harm in trying it to see how YOU get on.
Another alternative is the excellent free Opera click here. G