Hi merricat 
[s]Uploading pictures with a sandboxed Firefox is causing me problems as well.
But I am not sure if it is a general problem, or caused by the settings I use.
The other problems you descripe are new to me.
Have you tried to delete the contents of the Sandbox ?
Avast - Settings - Tools - Sandbox - Customise - Webbrowsers - Maintenance - Delete contents.
When the Sandbox is still in use, you can not delete it. A reboot will solve that.[/s]
Greetz, Red.