If you get the following error while using Google’s Gmail Service (gmail.com):

Oops… the system encountered a problem (#104)

..then you should try deleting your browser’s cache and everything will be resolved.

This appears to be a local problem and not on the Google servers as data is pushed via cookies to the local machine using the web service.

In order to delete your browser’s cache in Firefox click Tools->Clear Recent History then select “Everything” in the dropdown menu and then deselect everything except for “Cache” in the below list box.
Click “Clear Now” to complete this task.