I just discovered a slight difference between Firefox 5 and previous versions.
If I set a download running, then switch to another window (using the pager in fvwm), I can continue working. In previous versions, when the download was finished, a dialog box would pop up in the current window to tell me it was done. I could ignore it and continue working.
In the new Firefox, when the download is finished, I am suddenly forced back to the window with Firefox in it, then the dialog box pops up. This is irritating. So irritating it made me search for a change.
I found the necessary configuration changes. Pull up all the configuration with "about:config". Filter on "browser.download.manager", find "browser.download.manager.showAlertOnComplete", right click on it and toggle the value.
Great. The dialog box no longer pops up. But focus still switches back to the window with Firefox in it. Not good. Actually, really very irritating.
I looked through the rest of the config items, but could not see anything that would help.