Why is download from Firefox so slow?
Remove any unnecessary add-ons. Always keep Firefox up to date. Close any tabs that aren't necessary. Firefox's browsing and download speeds are slowed when there are too many tabs open.
Does Firefox limit download speed?
Firefox doesn't have a way to throttle the download speed. You will have to use an external download manager to get extra features like that. You can use the FlashGot extension to integrate an external Download Manager in Firefox.