Never had this problem with Firefox. Maybe a clean installation of Firefox will help?
1. Backup your bookmarks.
2. Uninstall Firefox.
3. Go to your Application Data folder by typing %appdata% in the address bar of Windows Explorer.
4. Rename the Mozilla folder to something else (like Mozilla-bak).
5. Browse one folder up to and open the Local folder: C:Users[username]AppDataLocal.
6. Also rename the folder Mozilla to something else.
7. Optionally you can also remove the Firefox registry keys: Open the Registry Editor (Start -> Run -> regedit) and navigate to HKEY_LOCAL_MACHINESOFTWARE (HKEY_LOCAL_MACHINESOFTWAREWow6432Node in x64) and rename all keys that start with Mozilla.
8. Install the latest Firefox from http://www.getfirefox.com/.
9. Restore your bookmarks.
Hope that solves your download problem!
