I did forget to say, that this has started since 38.0.1

Thank you for submitting your crash report(s). Those go into a database for triaging and prioritization, but for personal assistance, it would be great if you could post your crash report ID(s) here. You can find those on the about:crashes page as described in the last section of the support article "Firefox Crashes".

If Firefox won't stay "up" long enough to copy the list, have you tried 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.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

While you are troubleshooting this, it might be helpful, to avoid unnecessary pain on sites where Flash is not actually essential, try setting Flash to Click-to-Play ("Ask to Activate"). This will delay Flash from starting on a page until you approve it.

To set "Ask to Activate", open the Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. Look for "Shockwave Flash" and change "Always Activate" to "Ask to Activate".

When you visit a site that wants to use the Flash, you should see a notification icon in the address bar and one of the following: a link in a black rectangle in the page or an infobar sliding down between the toolbar area and the page. You can ignore the notifications if Flash is not important for that site.

Not a solution, but hopefully will help a bit.

Okay. Assuming the crash ID is the UUID in the report, it'll be this:

927fc6ea-6ed7-4b60-964c-bf58f2150519

Hmm, there isn't much info available about this crash signature:

nsStreamListenerWrapper::OnStartRequest(nsIRequest*, nsISupports*)

In the report, the crashing thread lists this function over 100 times in a row. That doesn't seem normal to me, but I don't know what page or process needs to open so many connections.

Can you associate it with any particular page or kind of content?

Did you have any better results in Firefox's Safe Mode?

No, not really, but I have gotten better results by clearing the cache and cookies, I haven't yet gotten a crash, but I'll keep you posted.

It seems to have stopped the crashes, but the window still loses focus, and occasionally flashes on the taskbar.

When Firefox loses focus, is there only one Firefox window?

There was (is?) an old bug in the Flash plugin where initiation of protected mode could cause focus to change between windows. The protected mode feature of the Flash player plugin has security benefits, but seems to have compatibility issues on some systems. There is a hidden setting to disable it if you want to try it:

(A) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(B) In the search box above the list, type or paste flash and pause while the list is filtered

(C) Double-click the dom.ipc.plugins.flash.disable-protected-mode preference to switch its value from false to true.

This might not take effect until all Flash has been unloaded for a few minutes, or you close Firefox.

No, there's only one firefox window.

You can try to disable McAfee SiteAdvisor.

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, I have had a stroke of genius, and identified the issue myself.

It was a bugged (or broken) extension, designed to force youtube to use the flash player rather than the HTML5 player, as the HTML5 player was bugged itself at the time.

Disabling it did the trick.

Aha! There are a couple of extensions for that, so if one doesn't work any more, you could try the other.

• https://addons.mozilla.org/firefox/addon/youtube-flash-video-player/
• https://addons.mozilla.org/firefox/addon/youtube-flash-player/

Or if you no longer need it, disabling or removing is good.

Well that first one there, is the one that caused the problem in the first place, but the HTML5 player is working now, so I won't need it. Thanks for the help though.