User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:56.0) Gecko/20100101 Firefox/56.0 Build ID: 20170704030203 Steps to reproduce: Pin multiple tabs with YouTube videos. Restart the browser (doesn't matter how). All tabs begin to play their videos even if they weren't activated. Happens with Safe Mode too. First observed within the last week, but could be an older issue.

I suspect it could be related to "lazy-loading" the pinned tabs. They used to appear simultaneously with the main browser UI, but now they load after the startup blank page and everything else is loaded.

I'm not worried about this issue because it is the same behaviour as previous Firefox versions. (In reply to smartfon.reddit from comment #0) > Pin multiple tabs with YouTube videos. Restart the browser (doesn't matter > how). All tabs begin to play their videos even if they weren't activated. > Happens with Safe Mode too. First observed within the last week, but could > be an older issue. Why would someone want to do that?

Same happens on Firefox 56 release now. It's different than on FF55. Previously a small icon appeared on each tab, allowing user to play its content in background on demand. Additionally the same takes place when browser.sessionstore.restore_on_demand is set to false, described in Bug 1381114

Just adding that on Firefox Nightly, version 61.0a1 (2018-03-18), this problem is still occurring. (In reply to Anthony Jones (:kentuckyfriedtakahe, :k17e) from comment #2) > I'm not worried about this issue because it is the same behaviour as > previous Firefox versions. > > (In reply to smartfon.reddit from comment #0) > > Pin multiple tabs with YouTube videos. Restart the browser (doesn't matter > > how). All tabs begin to play their videos even if they weren't activated. > > Happens with Safe Mode too. First observed within the last week, but could > > be an older issue. > > Why would someone want to do that? To respond to this: I use YouTube a lot as a source of music or video to play while doing work. As a result, I have a lot of YT tabs open. I found an album on YT that I really liked the beginning of and didn't want it to become lost in my other YT tabs. To avoid creating another window, I pinned the YT video that contained the album. I use browser windows as different work spaces. e.g., I will use one window for YT content, another window for school work, and another window for development documentation. At the very least, if this is a simple fix, having a flag I could activate to disable autoplay on pinned tabs would be extremely useful to me if Mozilla wants to keep the original, and currently default behavior.

Settings this TRUE solves the problem: browser.sessionstore.restore_pinned_tabs_on_demand