Try disabling graphics hardware acceleration. Since this feature was added to Firefox, it has gradually improved, but there still are a few glitches.

You might need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.).

Then perform these steps:

• Click the orange Firefox button at the top left, then select the "Options" button, or, if there is no Firefox button at the top, go to Tools > Options.
• In the Firefox options window click the Advanced tab, then select "General".
• In the settings list, you should find the Use hardware acceleration when available checkbox. Uncheck this checkbox.
• Now, restart Firefox and see if the problems persist.

I noticed that you have Shumway installed. Shumway isn't ready at the moment and Mozilla is looking to move away from Adobe. I haven't tried Shumway yet, but I'm guessing it doesn't work with YouTube, etc.
Another thing noticeable is that you're using Firefox ESR 24.4.0. Is there a reason you are using ESR as opposed to the general Release channel at 28 with 29 coming out really soon?

Sadly that did not help. Trying to view a programme offered by ITV under their itvplayer led to a screen telling me to update the Adobe flash player although I already have the latest one. The Firefox version I have is as far as I know the latest one for Linux: the Firefox website confirms it is up to date. Looks as if in order to see videos I would have to boot into Windoze instead. I ws just trying Shumway to see if it helped by intercepting the calls for a flash player. It appears not.

Note that some sites can use the built-in HTML5 media player to play videos in a video tag if you disable the Flash plugin. If you enable support for H.264 via GStreamer by toggling the media.gstreamer.enabled to true then that might work on YouTube. This pref is still false in the Firefox 24 ESR version, so you may have to update to the current Firefox release where this pref is true by default.

Adobe still supports Linux but only with security updates for the old Flash 11.2 with 11.2.202.350 being the latest version.

I set Flash to "Ask to Activate" as Flash will not play on page until you allow it.

I have Flash 11.2.202.356 and it is set to "Ask to Activate". It still crashes e.g. when trying to view a Youtube video. I have updated to Firefox 29 for Linux.

The advice to disable Flash and use the built-in, now I'm using Firefox 29, appears to work at least for Youtube. Thanks!

Hello,

linux flash plugin 11.xxxx appears to be incompatible with firefox 29 problems encountered are 1) very very slow on some video , may be some video never start 2) some displaying are truncated or unreadable

all problems disappears when using google chrome in place of firefox ( it seems that chrome has it's own embedded flash plugin)

it is mandatory that mozilla team works on this problem, now on ubuntu 14.04 firefox becomes unsusable

also an effort is needed to make firefox fully compatible with windows 8.1 (actualy dpi scaling is unusable, arrows in menu are enormous and pixellised)

petitlou60:

I can confirm that Flash 11.2 (latest version) works fine with Ubuntu 14.04. Also, please create a new thread at /questions/new as this question is solved. Before you create a new question, have you tried to disable Flash and use the built-in HTML5 Player as suggested by cor-el above?