I have the same problem with Android 4.4.

I've checked all my network settings. All my other machines has no problem with the network, just the Linux OS.

No ideas?

Android on an Android device, or Android in a vm on Windows?

"Server not found" suggests a DNS issue. Is Firefox configured to use a proxy or otherwise use different DNS from other browsers on the same system that can access those sites?

Yes, Android on a Samsung Tablet. I have the same problems with Firefox installed on Lubuntu 14.04 (64bit) as a VM on a Windows 7 (64bit) host. I've checked already all my network settings, worked through the support pages mentioned above but no solution so far. On both systems there's no proxy set. I tried static IP settings as well with the DNS set to the IP of my router as all the other machines in my LAN. The curious thing is, that I have these problems with the mozilla pages only. All other pages I tried working without issues?

For Linux, do you have any extensions preinstalled that could possibly block sites? Some have obvious name like Blocksite, but others may be less obvious. Try disabling anything nonessential on the Add-ons page. Either:

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

In the left column, click Extensions. Then cast a critical eye over the list on the right side. No extensions are included with standard Firefox; all are optional. If in doubt, disable.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Any improvement?

For Android, it seems unlikely that you would have unknown add-ons, but see: Uninstall Firefox for Android extensions.

I've checked the Add-ons. Under Android there's just a "OpenH264-Videocodec" from Cisco and one "Look for Firefox-Add-ons >" to call up the Add-ons page (I have the german firefox installed maybe the correct name differs a bit in an english version). Under Linux there's only one preinstalled Add-on named "Ubuntu Modifications". I deactivated this one, unfortunately no help.

Does firefox have any logging or debug function? Tomorrow I hope I have some time to look for these functions.

You could check the two consoles for error messages, but I'm not sure you'll find enough detail there:

• https://developer.mozilla.org/docs/Tools/Web_Console
• https://developer.mozilla.org/docs/Tools/Browser_Console
• https://developer.mozilla.org/docs/Tools/Network_Monitor

Thanks for the links, I didn't know these tools, learning never stops ;-) I've called up the firefox addons page again with activated monitoring. I attached some screenshots with the output of the consoles. I suspect a dns issue, but I've couldn't find the reason till now???

Thanks for the screenshots, but they don't resolve the mystery.

If it's DNS... Do you have any other browsers to try loading the site or any way to "ping" addons.mozilla.org:443 from your system command line?

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

• https://support.mozilla.org/kb/firefox-cant-load-websites-other-browsers-can
• http://kb.mozillazine.org/Error_loading_website

...after a lot of pings, settings and tracing, I've figured it out. As suspected the DNS... and after all the solution is quite simple... I tried some different DNS settings and had success with the DNS server set to google's 8.8.8.8 . Up to now on my Windows I had my router's IP set as the DNS Server. As my router isn't a DNS server of course, it forwards the DNS request to the DNS server assigned by my ISP. With Windows I had no problems so far, that's why I've tried these settings on my other systems as well. But Linux and Mac doesn't like it for some reasons. On all three systems I set my router/firewall as "Gateway" and google's DNS server on 8.8.8.8 as "DNS server" , now it works. Maybe I'll found out one day why Mac and Linux doesn't accept the same DNS settings as windows?

Thanks for your help, it helped to figure this one out. Rgds Pete