Install the DownThemAll download manager extension to Firefox. DownThemAll features an advanced accelerator that increases speed up to 4x and it allows you to pause and resume downloads at any time, including resuming interrupted downloads from where they were stopped.
In my experience splitting up the download into 4 parts, which is what DownThemAll does, reduces the download speed to about 5-10% less than the maximum speed that your Internet Service Provider can provide. If you are getting a download rate of 26kbs, then that is not DownThemAll's fault. Axel and aria2 (command line high speed download client) are two download accelerators that you can install from the Ubuntu Software Center if you want to try something else, outside of your web browser.