It's in the same window but not displayed? Could it be in a different tab group? If you press Ctrl+Shift+e, Firefox should display thumbnails of the tabs in the current window within one or more rectangles (groups). If you discover the missing tab sitting in its own separate group, you can drag it into the main group with the others. Press Ctrl+Shift+e again to switch back to the normal display.

I'm puzzled, though, that it is recreated after you've closed it. I wonder whether one of your add-ons might be responsible. What's in the tab (assuming you can get to it)?

By the way, your More System Details shows a user.js file in your profile folder (personal settings folder). Unless you created that file yourself, it may contains settings from an add-on or external software that override your own preferences. You can check and remove the file using the steps in this article: How to fix preferences that won't save.

If you find a file named user but not user.js, Windows may be hiding file extensions from you. To work with files as accurately as possible, I suggest unhiding them. This Microsoft support article has the steps: http://support.microsoft.com/kb/865219.

Thanks! I guess something I downloaded added a bunch of Funmoods options so I just erased them.

I have been having this same problem. When I go to close Firefox, I get the message asking if I want to close the 2 tabs, even though I only see one. To view the hidden tab, close the currently open tab. The hidden tab will reveal itself.

This just happened to me and the URL of the hidden tab was:

http : // s . ytimg . com / yts / jsbin / html5player-vfliqjKfF.js

(Spaces added so as to not create a click-able link.)

Hope this helps anyone with the problem and anyone researching the issue.

Hi DodgeRules, normally scripts do not open in their own tabs, nor are tabs normally invisible.

Do you see this problem often? Is it always a Youtube script (ytimg)?

If instead of closing the tab you check for multiple tab groups (Ctrl+Shift+e will open and close the tab groups screen), is the script in a second tab group?

I don't see it always, just every once in a while. It does always seem to be the Youtube script. This script is most definitely in its own hidden tab.

I was able to have it happen again. I had a Youtube video playing and then pressed the Ctrl-Shift-e keys to bring up the group. The video was playing in one tab while the group showed a 2nd tab containing the script. When I was finally able to see the hidden tab in non-group view, the tab had a "Sandbox" title.

The word "Sandbox" makes me thing of the Protected Mode feature of the Flash Player plugin. This feature splits up Flash into multiple separate threads with different permissions, to help reduce the security risk of playing Flash media. But it is a bit buggy and not all users are able to use it.

If you have not already disabled this feature for other reasons, you could try disabling it to see whether that resolves this hidden tab issue. The following pages provide different ways to do that:

• Adobe support article under the heading "Last Resort": Adobe Forums: How do I troubleshoot Flash Player's protected mode for Firefox?

• Manual steps: https://support.mozilla.org/questions/968190?page=5#answer-509209

• Batch file to automate the manual steps: https://support.mozilla.org/questions/982093#answer-518078

On third thought, the script file name references the HTML5 player. Have you opted in to the HTML5 trial?

https://www.youtube.com/html5

I went to the HTML5 player link and I had not opted in to the trial. I then went to play the "Lorde - Royals (US Version)" video and tried the CTRL-Shift-E trick. The hidden tab was there pointing to the same location as always. I selected it and this time the Sandbox label was not there.

I submitted a report to Youtube using the right-click on the video and selected Report a Playback Issue. Not sure if it will do anything.

I appreciate all your help. I just wanted to make sure it wasn't a Firefox issue.

I had the same problem. I made a research through my add-ons and then disabled 'Download Youtube Videos as mp4' (ver. 1.7.18) That resolved the problem.

Hop I helped you.

Recently I started having this problem both at home and work. I installed some new programs and I suspect them. The hidden tab is always

http : // s . ytimg . com / yts / jsbin / html5player-vfliqjKfF.js

I copied its address with Mozilla add-on Copy All Tabs

Yesterday I installed All Tabs Helper add-on and started to check opened tabs and to close this pest through it. Today it just stopped being visible in the listed tabs in All Tabs Helper.

I installed NoScript too but even if in the beginning it disabled the hidden tab (script) today the problem is active again.

I don't have Download Youtube Videos as mp4, I have DownloadThemAll, Ant Video Downloader and Download Flash and Video add-ons. I disabled the last two but this didn't helped.:(

By the way, your More System Details shows a user.js file in your profile folder (personal settings folder).

How could I find where this folder is?

Hi vessto, this article describes how to check for a user.js file: How to fix preferences that won't save.

To confirm that it is one of your extensions, could you try the site in Firefox's Safe Mode? That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled (Flash and other plugins still run)

In the dialog, click "Start in Safe Mode" (not Reset)

Does that eliminate the extra tab? Of course, it also blocks all the downloaders...

Occasionally we learn about an extension that hides itself from the Add-ons page. This ability to hide is removed in Safe Mode, so you may want to check again to see whether anything extra shows up.

Thank you for the reply!! First checked in the folder, no such file exitst.

The safe mode gave results. The tab didn't appeared. It usually appears from livejournal . com, in my page where I reposted videos from YT. Also from some time ago no matter how much I'm in livejournal . com and which page I visit, the LJ tabs I'm in never load fully, they continue to spin endlessly. Now in safe mod they at last loaded fully.

So it seems an extension (or plugin) causes that but I have many extensions and it would be hard to find out which is the violator.:/

I remembered I have my add-ons on screenshots so compared to the ones in safe mode no new add-on or plugin. I have one more video downloader, for VKontakte . ru, maybe should disable it too.

Nah, scratch this above, it seems it just needed time to appear in safe mode too.:(

What is also strange is that the players on my PC also started to open slower recently. Both AVG Free and Malwarebytes never warned me about any threat.

Hi vessto, there may be some glitch loading the script into the page itself that is causing it to load in a new tab. But at least you can see it, it's not hiding in a second tab group. If I load that address directly, I get a tab full of text (the script content), so it's strange that yours is blank.

Do you use any security software with script filtering features that could be changing how Firefox retrieves that one?

In case you have a systemic infection, this article lists some additional scanners Firefox users have found helpful: Troubleshoot Firefox issues caused by malware.

It hadn't text in safe mode but in normal mode it is full again (I attach new screenshot)

I suppose AVG Free has some processes that work online. But the free version hasn't support section so I couldn't ask.:/

There are some pop-ups in Mediafire that can't be stopped neither from Mozilla pop-up blocker nor from AB+. These pop-up even says it is Mozilla survey. But they are from some months and this .js appeared soon. Maybe I should contact LiveJournal admins to ask for the tabs that can't load. I'm not sure if this is related to the hidden tab though.

Hi vessto, is it possible to link the tab to a particular page? If you can post a URL or two, a volunteer could try it out.

It happens in my personal LiveJournal so I'd like to provide link in pm if this is possible.

Hi vessto, if you click a user name, you can send a message from there.

Does it occur if you visit the page in a private window (Windows: Ctrl+Shift+p) where you are logged out and have fresh cookies?

(If it only occurs after you've logged in to administer the site, there may be some extra features that will be hard to replicate.)