Mozilla Suite : FAQs : Download
From MozillaZine Knowledge Base
File downloading has been very slow lately, why?
When you have a large list of download history, Mozilla becomes slow in starting and finishing file downloads. To clean the list, open the Tools menu and choose Download Manager. Select the entries you want to delete (hold down the Shift key for continuous selection, and the Ctrl key for non-continuous selection). Then click Remove from List.