Rated 4 out of 5 stars

With this addon, I can jump to each section without dragging the scrollbar. This is very useful on such cases: I know the page includes a specific topic but I don't know where it is. Different from in-page search, existing summaries (headings) are shown visually beside the scrollbar.

For Japanese (and Chinese, Korean) people, I recommend you to customize the hidden preference "extensions.contentscrollbar.textlength" to "firstTenCharacters". By default it is "firstThreeWords" but it works same as "entireText" for CJK webpages because CJK terms don't include white spaces to split words in most cases. In such cases, "firstTenCharacters" will work better than the default. (I want to enlarge the number of max characters than 10, but sadly it can't be done...)

One request, it will make me happy that auto-hide feature for this addon. Even if I want to see contents near the scrollbar, there are links made by this addon so sometimes they cover the contents. If those links are hidden (or shown with very small width) by default and become expanded to full size when I move my mouse near the scrollbar, they won't interfere with my view.

By the way, I currently use this with another addon "Custom Buttons" ( https://addons.mozilla.org/firefox/addon/custom-buttons/ ) to add a toolbar button to toggle functionality of it.
My button "Toggle Content Scrollbar" is defined with following codes:

Initialization Code:
var key = 'extensions.contentscrollbar.enabled';
var Prefs = Cc['@mozilla.org/preferences;1'].getService(Ci.nsIPrefBranch);
this.type = 'checkbox';
this.autoCheck = false;
this.checked = Prefs.getBoolPref(key);

Code:
var key = 'extensions.contentscrollbar.enabled';
var Prefs = Cc['@mozilla.org/preferences;1'].getService(Ci.nsIPrefBranch);
Prefs.setBoolPref(key, !this.checked);
BrowserReload();
this.checked = Prefs.getBoolPref(key);

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Japanese version:
これがあれば、スクロールバーをあちこちドラッグしなくても、ページ内の各セクションに一発でジャンプできます。そのページに何か探している情報が含まれている事は分かっているけれども、どこにあるかは分からない、という時などに便利だと思います(自分は、技術文書などでそういうことがよくあります)。ページ内検索と違って、ページ内にある要約となるテキスト(見出し)がスクロールバーの横に表示されるという点が特長です。

日本語圏や中国語圏、韓国語圏の人は、隠し設定の「extensions.contentscrollbar.textlength」を「firstTenCharacters」(先頭10文字だけ表示)にするのがオススメです。初期値の「firstThreeWords」(先頭の3単語を表示)は、単語の区切りに空白文字を使わない言語では「entireText」(全文を表示)と変わらない結果になってしまいますので。(ちなみに、文字数を10より増やすということはできないようです。残念。)

なお、自分はこのアドオンの機能の有効・無効を簡単に切り替えられるように、Custom Buttons( https://addons.mozilla.org/firefox/addon/custom-buttons/ )というアドオンと併用しています。新しいユーザ定義ボタン「Toggle Content Scrollbar」を作成して、Initialization CodeとCodeの内容を上記の通り設定しています。

This review is for a previous version of the add-on (0.1.0.1-signed.1-signed). 

Try the new 0.2.0 version (https://addons.mozilla.org/firefox/addon/content-scrollbar/versions/?page=1#version-0.2.0) that allows you to
- Add a toolbar button and let the markers show/hide for the current tab easily.
- Set extensions.contentscrollbar.enabled = minimized to see the minimized markers