Do you have any extensions running ? If you can't execute javascript correctly or you have extensions you might try disabling the extensions one at a time.

I use firefox most of the time, I can't say I have ever had this happen so it would be a shot in the dark at the cause.

I've been using Chrome with zero problems in Concrete for months. What did you see that Chrome was doing?

The only extension running was Microsoft .NET Assistant. I've disabled it but the behaviour is unchanged. The Firefox Error console is full of things like "Ruleset ignored due to bad selector" and "Error in parsing value for 'filter'. Declaration dropped."

Chrome has always been a bit weird in editing C5. Viewing is fine. It also has the behaviour that clicking on a file in C5 File manager shows no menu of options. This is on a Vista machine btw. Perhaps the problem is there.

Dump your install of Firefox, and clean everything up. Make sure you get rid of the stuff stored in your personal profile i.e. C:\Users\<user>\appdata\roaming\Mozilla

Not sure on the location since I am on xp at work. Then reinstall firefox, you might do the same for chrome. It sounds like your install might be broken.

Update: Interesting, my work machine just threw out the same errors for the .net assistant and Microsoft presentation. I wonder if it is a windows update that is causing browser problems. However all of the functions on my c5 install seem unaffected.