This is the screen I get

What type of device is that screenshot from? Doesn't look like a desktop version of Firefox.

Of course, it is.

FF open this as a 2nd window.

I'm using w8.1 (x64) with ff 38.0.1

Do you have multi-process tabs (e10s) enabled in Firefox?

You can check this prefs on the about:config page and if user set (bold) then reset the pref via the right-click context menu to the default value.

• browser.tabs.remote.autostart

No it isn't activate.

Status: default Value: false

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

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

I'm just now in Safe mode to check it out, but this error-window still appears from firefox, thou my add-ons or hardware accelerator may not be the problem.

Boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/

Do a malware check with several malware scanning programs on the Windows computer. Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  http://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked
• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware

Well, my computer is not infected.

I uninstalled Firefox and deleted every folder from mozilla.

Using mozbackup just with add-ons and bookmarks after reinstalled Firefox, used to help me to get this fixed.

Fortunately, it helped to fix this tab-crash-issue and kinda boost my Firefox.

I only upgraded Firefox continuously since a couple of years, btw. This might be the cause.

Many thanks to the moderator cor-el, who tried to help me out very quickly with this problem.

This is some sort of malware. As noted above, it doesn't look like any sort of Firefox screen, but every time I go to a new tab, this thing pops up in a new window.

However, Windows Defender doesn't catch it, and a fully updated version of MalwareBytes didn't either. Still, it's clearly malware, and I suspect that many more people will get this in the coming days.

mingusal said

This is some sort of malware.

I don't think so. This screen popped up right after updated to the new firefox-version. And after updated, opened my browser, didn't go on any site and this screen came up without doing anything.

I didn't have this before.

But however, the clean uninstall and new install of firefox was more than helpful to me :D