By sidebar video, do you mean the ones listed down the right side of the player page? Hmm, I don't notice this problem.

Let's take it in ultra-slow motion:

You slowly right-click the title of the video, and Firefox drops the context menu. If you pause here, can you verify that Firefox didn't navigate the current page to a new video at this point? It should still be playing or paused, however you left it before right-clicking the link.

Then you slowly left-click the top item (Open Link in New Tab) and Firefox sends the URL to the new tab. I think that normally would open in the background, so you still are looking at the current tab, playing or paused, not changing videos. At least, that's what I would expect.

(If it matters, I never log in to YouTube unless I need to reply to a comment on one of my videos, so all testing was "logged out.")

What do you mean by this part:

Also (not as important as above) - why do videos need to be opened twice in youtube?

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems