What versions of Chrome/Firefox? Check you've got the latest one (
3.6.13 for Firefox)
If you have the latest versions of both browsers:
(*) Try Opera? Quick fix
(*) Describe the problem a bit better - What would you expect to happen? What actually happens? Does this happen every time, or only sometimes?
(*) Try looking at the error console in Firefox - Ctrl+Shift+J, or Tools>Error Console