See https://support.mozilla.org/en-US/kb/compacting-folders You will not see anything in there about archiving I am fairly sure, at least there was not when I first wrote it about 10 years ago. I do recommend you read the part about complications and then move on to this https://support.mozilla.org/en-US/kb/compacting-folders-potential-complications There is no relationship between compacting and anything being full, and that is why I am directing you to background information. So once you get some background, do you have any NSTMP folder in your local folders?