Type about:addons<enter> in the address bar to open the Add-ons Manager. Hot key; <Control>(Mac:<Command>) <Shift> A)

On the left side of the page, select Plugins. Make sure the ones you want to use are set to Ask or Allow.

Go to the web page. Once the page is loading, mouse to the address bar and Left click the icon. A window for displaying site information should come up. Select More Information. Now select Permissions. In the menu, Make sure the ones you want to use are set to Ask or Allow.

Thanks for the reply, but it didn't work. Problem persists.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

check this:

• see if there are updates for your graphics drive drivers

https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)

https://forums.adobe.com/message/4468493#TemporaryWorkaround

• disable hardware acceleration in the Flash plugin

https://forums.adobe.com/thread/891337 See also:

• http://kb.mozillazine.org/Flash#Troubleshooting

What happens if you try to load that MP4 directly? https://video.twimg.com/ext_tw_video/640920500705861632/pu/vid/240x180/vs9aHUFkVV4z5w5V.mp4

On Windows XP, Firefox does not have the necessary code to play MP4s itself, a plugin is needed. Twitter may not be handling that in the best way, so I guess the question is whether it's possible to help it along by disabling a setting somewhere so Twitter uses Flash or another plugin instead.

It's difficult to me to get hands on XP myself, so hopefully someone else can experiment.

Thank you both for your time and effort.

FredMcD, the situation remains the same in Safe Mode.

jscher2000, the mp4 video doesn't play on its own, so I guess it's a matter of handling the mp4 videos on Firefox. This is rather unfortunate, as Chrome plays these videos fine. I hope there is a solution, as I can't change the OS for various reasons.

Thanks again for your help.

Hmm, when you opened the MP4 directly, I would have though the VLC plugin would jump in and play it. Could you check whether any of your plugins handle MP4? You can check that as follows:

Type or paste about:plugins in the address bar and press Enter. Then use Find (Ctrl+f) to search for mp4 in the page. Since I don't use VLC but I do have QuickTime, for me QuickTime would handle a "stand-alone" MP4.

But in the case that the page uses an HTML5 player without checking whether Firefox can play MP4 in that player, which is a problem on XP, the plugin might not kick in. Hopefully someone knows of a technique to lead the page to use a plugin instead.

On Windows XP you would have to set media.directshow.enabled to false to disable the HTML5 media player and possibly have the website switch to a Flash based player. The error message would suggest that Firefox tries to use the HTML5 media player and that is failing.

Firefox would normally only use a plugin if an OBJECT or EMBED tag is used to play the file.

Thank you all for your inputs.

jscher2000, I triwd to install the Quicktime plugin, but FF doesn't support its previous version that works with XP and the latest update requires Windows Vista or higher.

cor-el, setting media.directshow.enabled to false doesn't solve the problem.

I guess I will have to continue using Chrome for Twitter, which isn't really something I am happy doing.

Thank you again.

Same problem here. I'm running Ubuntu Linux 14.04.3 Firefox 40.0.3 Started about the same time as the original poster.

The video.js library used by Twitter is supposed to "fall back" to Flash when the HTML5 player doesn't support the available formats, but it's failing and I can't tell why.

I tried to develop a user script to embed a plugin but I just can't get it to work correctly (for example, zoomed too much, no controls, etc.). The best I could do is display a link to open the media in a new tab and if you have a plugin that plays MP4s, such as QuickTime or VLC, that should play the media in the new tab.

Maybe someone else can take it the next mile.

Note: To install the user script, you need the Greasemonkey extension first:

https://addons.mozilla.org/firefox/addon/greasemonkey/

Then see:

https://greasyfork.org/en/scripts/12427-twitter-html5-video-error-to-link

Thank you so much jscher! This helps a lot, at least won't have to use a different browser to watch a video. Thanks for your time and effort.

I also have this problem as well. The worst part is the method in which you needed to disable hardware acceleration to see some videos also didn't work. This Greasemonkey script looks to be the only way you can view videos on Twitter properly while on Firefox.

If it's not much, when Twitter recognizes a video link, and the Greasemonkey script kicks in, it will tell you which program or plug-in is supposed to handle it. Mine is supposed to be handled by the QuickTime Plug-In version 7.7.6. I never got any notifications to say to enable QuickTime. That could be a clue.

Until further notice, for all Firefox users having this problem, it is highly recommended that you download Greasemonkey as well as jscher's script.

• プラグインを無効化
• プラグインを最新版に更新
• Safe modeで再起動
• about:configで値をチェック

以上のようなことをしても解決しない場合、jscher2000氏のスクリプトが大変役立ちました。

1.Greasemonkeyをインストールする [https://addons.mozilla.org/ja/firefox.../greasemonkey/] 　GreasemonkeyはFirefoxユーザーが拡張機能を利用しやすくする為のアドオンです。これ自体が動作するのではなく、このアドオンを利用してスクリプトを編集し各ユーザーに合わせたブラウジング環境が作成可能なツールです。

2.Twitter HTML5 Video Error to Linkをダウンロードする [https://greasyfork.org/en/scripts/124.../12427-twitter-html5-video-error-to-link] 　この問題だけを解決する為のスクリプトです。Greasemonkeyをインストールしてから上記リンク先の"Install this script?"と記載された緑のリンクをクリックすると自動的にGreasemonkeyへインストールされます。

日本語では以下のスクリプトの編集が必要です

1.Greasemonkeyのメニューから"ユーザースクリプトの管理"を開きます。 2.切り替わったページの"設定"を開きます。 3.スクリプト設定のタブを選択し、"https://amp.twimg.com/*"にカーソルを合わせ、下の"このユーザースクリプトの編集"を開く。 4.13行目の"does not support video playback"を"再生できません"に書き換え保存。 5.3をもう一度繰り返して書き換えられたか確認。

成功していれば、Twitterで再生できなかったmp4動画の再生ボタンをクリックすると画像のようなリンクが現われます。クリックすると新しいタブで動画が再生します。ブラウザ上でTwitter以外ではmp4が再生できる状態（Quick TimeやVLCがインストールされている）が前提の解決法です。

円満な解決法ではありませんが動画のURLを表示させてコピー→新しいタブで開くという煩雑な作業がなくなりました。jscher2000氏を筆頭に参加された皆様に感謝します。

Firefox 42.0 is out and this problem persists,, Not too happy that Firefox and Mozilla don't care about us (10s of thousands) XP users to find a workaround - even if its just a setting to allow us to point to a user/system-installed codec

• sigh*

Doesn't work in Ubuntu Linux either, Firefox 42.0 Sure wish someone would figure out a fix for Linux!

I have this problem also. Seems like Mozilla is spelled Microsoft these days.

menothappy said

I have this problem also. Seems like Mozilla is spelled Microsoft these days.

It is a limitation of the old EOL Windows XP.

capot1948 said

Doesn't work in Ubuntu Linux either, Firefox 42.0 Sure wish someone would figure out a fix for Linux!

Actually HTML5 videos like those of Youtube and Twitter works very well on Linux.

You just need to have the appropriate Gstreamer packages installed.

I cannot say which ones specifically for you as Linux distros may split up the needed Gstreamer packages a bit differently.

Thanks James, when Firefox was updated to V43.0 videos started working again. Now possibly there were Ubuntu updates Gstreamer which I didn't catch also. But, happily it's working again.

James said

menothappy said

I have this problem also. Seems like Mozilla is spelled Microsoft these days.

It is a limitation of the old EOL Windows XP.

Ya know, you CAN install a avc/aac/mp4 (etc) codec in WinXP yourself.. and Firefox _NEEDS_ to allow you to point to it / use it.