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I'm currently using a CentOS Linux release 7.2.1511 (Core) operating system on a Dell machine. I've been trying to watch YouTube videos with Mozilla Firefox ESR 45.1.1, but it never worked.

("It's necessary Adobe Flash Player to play this video. Download the updated version of Flash Player")

I downloaded it but nothing worked.
I also tried to install some Firefox Add-Ons for Flash playing (YouTube Flash Video Player and Enable Your Youtube Flash Player) but nothing worked.

Any suggestions?

Thanks!

While HTML5 should be the default player and not Flash, try using an add-on for Firefox to force it to actually use HTML5. Here are some add-ons that should do the job:

• YouTube ALL HTML5


• YouTube HTML5 Video


• YouTube HTML5 Player

To make sure your browser has complete HTML5 support, you should also update it to the latest version if you haven't already.

I just tested it on latest CentOS, it should work:

1. First, go to https://get.adobe.com/flashplayer/
   


2. 
   

   Select either yum or rpm is ok:

   
   
   

   

   
   



3. Click Download now button in right side:

4. 
   Software Install is not reliable, so we must choose Save File:

5. Open you terminal:

Navigate to ~/Downloads directory which hold the rpm you've downloaded just now, and do sudo rpm -i <.rpm file name> and sudo yum -y install flash-plugin:

[xiaobai@localhost ~]$ cd ~/Downloads/
[xiaobai@localhost Downloads]$ ls
adobe-release-x86_64-1.0-1.noarch.rpm
[xiaobai@localhost Downloads]$ sudo rpm -i adobe-release-x86_64-1.0-1.noarch.rpm
[xiaobai@localhost Downloads]$ sudo yum -y install flash-plugin

Close all firefox window and restart it, it should work now. Visits to https://www.adobe.com/software/flash/about/ should able to see your version:

Footnotes:

1. After rpm -i <.rpm filename>, it will make yum search flash-plugin work, and so yum install flash-plugin will succeed.


2. Press Ctrl+C to kill and wait for few seconds to try yum install flash-plugin again, if you see yum lock error.


3. Keep in mind that most of the popular YouTube videos shouldn't need Flash. Your question didn't state is it not working for all videos or only few videos. This is important statement since I have no problem to play YouTube videos in a freshly installed CentOS without flash player.


4. 
   Update your Firefox if still not working. Disable all of your Firefox Extensions/Plugins(except Flash Player) to test if still not working, because UA header might affect YouTube decision to give
   you flash or html5 code. Ensure Plugins page's Flash Player set to Always Activate for testing purpose. Log-out your account and start a New
   Private Window to test if still not working.

Adobe abandoned Flash for Linux years ago. Which is a good thing because Flash is a security nightmare.

If you really must play a flash video, the best method is to just download it and play it with mplayer or similar. Or convert it from .flv to .mp4 or something with ffmpeg or handbrake. There are several video-downloader plugins available for firefox that make it easy to download a video rather than play/stream it.

Alternatively, use google chrome or the open-source version chromium with the Pepper Flash Player. Note, however, that flash is and always will be a constant stream of security holes that may be used to compromise your system via a cunningly mal-crafted flash program.

I have no idea if it's pre-packaged for Centos or not, but it's packaged for debian as pepperflashplugin-nonfree - that contains a script which downloads google chrome, extracts the flash player and installs it for chromium to use (so you can get the flash player in chromium without google's other proprietary spyware).

Note that support for 32-bit has been dropped by chrome's pepper flash.

You can try the approach here:

https://www.centos.org/forums/viewtopic.php?t=4926

Basically, it's

sudo yum install flash-plugin

And some updating work/fix dependencies.

It's a possible threat to the security of your system. At your own risk, like many have stated.

Here's how I was able to view HTML5 YouTube videos in Firefox running on CentOS7 using the Gnome desktop.

Using the default Firefox install, press "alt" and "T" key to select "Tools" on the browser (using shortcut keys will show pull down menu if not active).
Then, select "addons" from the pull-down menu.
Once on plugin page, select "get add-ons" and did a search for HTML5.
There were many plug-in choices, apparently enabling both HTML5 and Flash to be viewed on YouTube. Several are "free" at the moment, but with varying degrees of data collection and possibly other annoying behaviors.
The plug-in installed automatically just by pressing the install button next to the plug-in. It did not require a browser restart.

As for which plug-in, the "f|5" plug-in worked for me. I picked it only due to it's popularity.