the_dinamo said

The YouTube stream stops constantly and starts loading. This happens only when the tab with the stream is not active, that is, the stream is on one tab, and I'm on another or in another window. In other browsers this is not, therefore the problem is in the browser itself.

That depends on your ISP bandwidth settings if it is to low of speed something will slow down in the process. As for other Browser unless someone from the same location has the same result on same Browser that will be hard to fault just a Browser. Do you have other addons installed?

That is a Firefox feature called Network Throttling to reduce network traffic from inactive (background) tabs.

You can set this pref to false on the about:config page to disable this throttling.

• network.http.throttle.enable = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• http://kb.mozillazine.org/about:config

There are other things that need attention.

The System Details list shows an outdated Flash 18 plugin that you should update.

• Flash 18.0 r0

You can uninstall the outdated Flash plugin and install the latest Flash version.

See this about uninstalling Flash players:

• http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html

You can download the latest NPAPI Flash plugin for Firefox on this page. See "Still having problems"

• http://helpx.adobe.com/flash-player/kb/installation-problems-flash-player-windows.html

You can uninstall the current Flash player version and (re)install the latest Flash version.

• http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html

You can download the latest NPAPI Flash plugin for Firefox on this page.

• https://get.adobe.com/flashplayer/otherversions/