By default Firefox displays only a visual notification when a download is completed. This visual sign appears in the browser’s download button and taskbar icon. As for the latter, the completed download is indicated by the disappearance of the progress bar. It is especially helpful when the browser is not in focus because you don’t have to open it just to find out if the download is finished. However, if you think that the notification isn’t prominent enough to catch your attention, then perhaps you want to add something like sound to the completed downloads. After browsing for a while in Firefox add-on repository, I found three add-ons that are capable of doing so.
Download Sound (v1.0.1) is a simple, restartless extension that automatically adds sound to completed downloads once it is installed. Additionally, clicking on its toolbar button lets you open the download list in a new tab. Unfortunately, you can’t use your own sound.
Download Status Bar (v184.108.40.206) offers tons of features to replace the Firefox’s default download manager. One of those is the ability to add custom sound for completed downloads. However, this function needs to be activated manually from the extension’s option page.
On the other hand, Modern Download Manager (v0.2.1.1) also provides a lot of options when it comes to downloading stuff from the internet. Just like Download Status Bar, it also allows you to use any sound you like. It’s not restartless so you have to restart browser for the extension to work.
Each extension above has been tested on the latest Firefox version at the time this post is published, which is Firefox v43.0.2, and all of them worked perfectly (note that I only tested the sound feature).