Quantumになって、ファビコンカスタマイズのアドオンが見つからず、残念に思っていましたが、
このアドオンでカスタマイズがまた可能になりました。使い勝手も良いと思います。

1）ファビコンをカスタマイズしたいブックマークのURLをコピー
2）アドオンの詳細画面でURLの欄にCtrl+Vでペースト
3）png形式の画像をcustom faviconの欄に選択した後
4）ファイアフォックスを再起動
5）ファビコンを変更したブックマークを選択してホームページを表示することで
新しいファビコンイメージが表示されます。

I know this is experimental, so feedback is important to improve it. When I first installed it, the add-on worked like a charm. Unfortunately, that's not the case anymore. For some reason, the favicons are not staying. There have been updates to Firefox since I installed it, so that may have to do with it. I hope the issue can be fixed because I really like adding favicons to certain pages.

Every time, need to be on the tab to see the new favicon. It is an overhead.
If it does this automatically, it would be nice:)

I am using Firefox 58. The add-on worked for a couple of times. Now I can not get it to work at all.

This ABSOLUTELY WORKS! Thank you for making this extension. It saves me so much time, no more chrome.css editing! Awesome sauce for those that love custom favicons.

新しいFirefoxで利用出来なくなった「Bookmark Favicon Changer」の代わりに使えました。
ただファビコンのエクスポート機能はないので特定のサイトのファビコンを利用したい場合は
現在も利用可能なChrome版「Bookmark Favicon Changer」のエクスポート機能を利用しました。

ファビコンの拡張子は「.ico」は利用できないようで「.png」なら使えます。

セーブしてもファビコンが変わらない場合は以下を再確認してみてください。

・「page title string」にはタブに表示されている正確なサイトタイトルを記入する。

・セッティング前にブックマークしていた場合は古いブックマークを一旦削除してリロードして
　再度ブックマークに追加してもう一度リロードしてみる等…

設定記入欄はFirefoxの仕様でクリック禁止になっていてクリックによるコピペが使えませんが
ショートカットキーによるコピペは使用可能です。
URL等を記入する際はショートカットキーでコピペすると楽です。

It doesn't work for Firefox 57 Quantum. there are no instructions anywhere for how it should work.

How to use? Cant see anything new in my Firefox.

I got this to work once I made sure the URLs were exact. It won't work on subdomains unless I set up an entry for each subdomain. It would be helpful have a favicon for a site and all it's subdirectories (eg http://*.example.com) and then override those where the specific URL is given. I'd also like to set up a favicon using the URL and a string that's in the path or parameter (which may also work for subdomains) so that when i have different sites open in web.archive.org I can differentiate them. The page title string isn't effective for any of the sites I need because they all use the main site name as the page title if they have one, so it would be helpful differentiate them by a common string in part of the URL.
Overall this is a huge help in trying to quickly find the tab I want when i have a lot open.

Does exactly what it says. Useful if you use the windows accent color in the titlebar and run across favicons with poor contrast

ne semble pas fonctiopnner

Great for use when you pin tabs without and favicon to identify. Webmail, chats etc.

Seems simple enough to use. Local site had no icon which was annoying since I have the page pinned so it was just blank essentially. Used their own logo from the page as new favicon and dropped in the URL. Then a quick refresh of the page and presto! New favicon set. THANKS!!

It simply doesn't work, at least not on bookmarks toolbar.
Update: you have to actually open that bookmark for it to update, so it actually works. Just not the way i expected it to.

I just downloaded it and it worked. Thanks!

It works for the first URL, but not on any subsequent ones.
Definitely not ready for prime time.
I finally got it to work on open tabs.
It does NOT work on bookmarks so it is of little value to me.

does not work

No idea what to do. Added the URL of the site, added a PNG for the favicon... nothing has changed, nothing has happened. Reloaded the tabs and the favicon stays the same. Doesn't seem to do anything.

I can't figure out how to use this thing. Basic instructions would be nice.

Thank you! This is a great little helper, that does what it says.