Styles

Example styles for userChrome.css/Stylish:

• hide some menu items
• show NN% label after each progress bar

API

You can use DownloadPanelTweaker:OpenDownloadWindow (“open old window”), DownloadPanelTweaker:ToggleDownloadSidebar (“toggle sidebar”) and DownloadPanelTweaker:OpenDownloadTab (“open in tab”) events to override commands, example:

window.addEventListener("DownloadPanelTweaker:OpenDownloadWindow", function(e) {
    e.preventDefault();
    // Put here some code to open alternative download window
}, false);

Internals

Compact downloads list style: https://github.com/Infocatcher/UserStyles/tree/master/Compact_downloads
Visible downloads count limit: https://gist.github.com/Infocatcher/5387328
Show download rate: https://gist.github.com/Infocatcher/5787749
Don't remove finished downloads: https://gist.github.com/Infocatcher/6452231

Additional tweaks can be done using following style (but [downloadPanelTweaker_paused] doesn't work without extension):

/* Download Panel Tweaker */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
@-moz-document url("chrome://browser/content/browser.xul"),
    url("chrome://browser/content/places/places.xul"),
    url("about:downloads"),
    url("chrome://browser/content/downloads/contentAreaDownloadsView.xul") {
    #downloadsListBox { /* Firefox < 20 or NASA Night Launch theme */
        width: auto !important;
        min-width: 0 !important;
        max-width: none !important;
    }
    .downloadContainer,
    .download-state > vbox /* Firefox < 20 */ {
        width: 58ch !important;
        min-width: 0 !important;
    }
    #downloadsSummaryDescription,
    #downloadsSummaryDetails {
        width: 58ch !important;
        min-width: 0 !important;
    }
    .downloadTarget {
        min-width: 5ch !important;
    }
    .downloadProgress {
        min-width: 20px !important;
        min-height: 10px !important;
        height: 10px !important;
    }
    .downloadProgress > .progress-bar {
        height: auto !important;
        min-height: 2px !important;
        max-height: 10px !important;
    }
    /* Paused downloads */
    .download-state[state="4"] .downloadProgress,
    #downloadsSummary[downloadPanelTweaker_paused] .downloadProgress {
        /* Firefox 35 and older */
        filter: url("chrome://mozapps/skin/extensions/extensions.svg#greyscale");
        /* Firefox 36+ */
        filter: grayscale(1);
    }
    .download-state[state="4"] .downloadProgress > .progress-bar,
    .download-state[state="4"] .downloadProgress > .progress-remainder,
    #downloadsSummary[downloadPanelTweaker_paused] .downloadProgress > .progress-bar,
    #downloadsSummary[downloadPanelTweaker_paused] .downloadProgress > .progress-remainder {
        opacity: 0.85;
    }
}