This may be completed unrelated, but it was a change in Firefox 36.0 and 36.0.1, so:

Firefox 36.0 apparently started issuing a second DNS query ("ANY" after "A") to get the TTL for DNS records. This created a lot of concern among DNS operators because it looked similar to a reflection attack and was disabled in Firefox 36.0.1 by changing the value of a preference. You can make sure you have that in effect as follows:

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

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

(3) If the network.dns.get-ttl preference is bolded and user set to true, double-click it to toggle it back to false

That may or may not have any effect on this issue, but if your DNS provider is dropping your requests because it suspects an attack, we could at least avoid that.

jscher2000 said

Firefox 36.0 apparently started issuing a second DNS query ("ANY" after "A") to get the TTL for DNS records. If the network.dns.get-ttl preference is bolded and user set to true, double-click it to toggle it back to false

Thank you for your answer. Unfortunately this is not related to by problem. network.dns.get-ttl is false (default) in my case. As I said, my issue is Firefox does not (trigger my OS to) send DNS requests at all hence no answer would be possible. Aslo, manually setting DNS does not hep for the same reason.

I can't think of a reason that Firefox would stop making DNS requests.

Perhaps you could file a bug and see whether the developers have some diagnostics you can set up to capture data about internal glitches when this happens.

https://bugzilla.mozilla.org/

Try to disable IPv6 (check for other possible causes as well).

• http://kb.mozillazine.org/Error_loading_websites

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

I was having this same exact problem for the last week. It started happening after Firefox updated. I tried a few different things that didn't help. I tried uninstalling and reinstalling which didn't help either. I finally fixed the problem tonight.

Here are the steps I took to fix it. Make sure you are logged into sync and hit the sync button to update it. This will save your bookmarks and user settings.

If you want to be able to restore your current session go into your main drive and click users, your user name, AppData, Roaming, Mozilla, Firefox, Profiles, open that folder and copy sessionstore-backups and save it to your desktop or somewhere you can remember.

After you have done that uninstall Firefox. Now restart your computer into Safe Mode.

Go into your main drive usually C on most computers and click users, your user name, AppData, Roaming. Click the Mozilla folder and delete it. Now hit the back button and click on Local. Find the Mozilla folder in there and delete it too.

You can now restart your computer and install a fresh copy of Firefox. Once it is installed you can log into sync and it will restore all your settings and bookmarks. 

If you want to restore your previous session close Firefox and make your way back to AppData, Roaming, Mozilla, Firefox, Profiles, open the new profiles folder (Example: abcd1234.default) and paste the folder you saved called sessionstore-backups in there.

You can now open Firefox and all your tabs will be there, and after a few mins all your settings and bookmarks will be restored by sync. You shouldn't have any more problems following this method. The pages no longer stop loading and it closes like it is supposed to.

Hi dedj1234, and other readers, there is an easier way to test both a clean installation of the Firefox program files and a clean settings folder, which preserves more data. I would try this first:

Create a new Firefox profile

A new profile will have your system-installed plugins (e.g., Flash) and extensions (e.g., security suite toolbars), but no themes, other extensions, or other customizations. It also should come with completely fresh settings databases and a fresh cache folder.

Exit Firefox and start up in the Profile Manager using Start > search box (or Run) -- type or paste the following and press Enter:

firefox.exe -P

Don't delete anything here.

Any time you want to switch profiles, exit Firefox and return to this dialog.

Click the Create Profile button, assign a name like March22, and ignore the option to change the folder location. Then start Firefox in the new profile you created.

It might take a while to determine whether the new profile is works better than your old one. If you want to import your bookmarks from your regular profile during that time, I will put the steps for that at the end.

If the new profile has the same kind of failures, it could be corruption of Firefox's program files. See the "Clean Reinstall" section below.

When returning to the Profile Manager, you might be tempted to use the Delete Profile button. But... it's a bit too easy to accidentally delete your "real" profile, so I recommend resisting the temptation. If you do want to clean up later, I suggest making a backup of all your profiles first in case something were to go wrong.

Clean Reinstall

(1) Download a fresh installer for Firefox 36.0.4 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(2) Exit out of Firefox.

(3) Rename the program folder

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(4) Run the installer you downloaded in #1. It should automatically connect to your existing settings.

When starting Firefox, you may need to use the profile manager to switch back to your regular profile if you didn't do that before.

Does Firefox run any better with the new program files?

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins

Cross-restore bookmarks: You can use the Library dialog (Ctrl+Shift+b or "Show All Bookmarks") to access the Restore feature. The trick is knowing where to find your bookmark backups in your regular profile. After clicking Choose File... you can paste this into the file name space and press Enter to drill down to your profile folders:

%APPDATA%\Mozilla\Firefox\Profiles

The semi-randomly-named folder that has your last backup is usually the one ending with "default". Click into that one, then into the bookmarkbackups folder. Here you can pick the most recently updated file to restore.

Important: a Restore wipes any bookmarks currently in your Firefox and replaces it with the contents of the restored backup.