You should be able to restart a download after you cancel it. There normally are two buttons: Cancel and Pause. After you cancel, you see a Restart button. (See Where to find and manage downloaded files in Firefox for more details.) Does this work?

I'll try this next time I have a download that's hung (which is another thing I hate - why won't they just reduce the timeout!).

I haven't tried this recently because I seem to remember it removing the download when the cancel button was pressed.