What flavor of Linux are you using? Are you using the Firefox from your distribution's repositories, or from mozilla.org?

We haven't seen this issue in Firefox v34+. So I am thinking this is probably an issue specific to the distribution you are using.

Thank you for your attention to my problem. I use Slackware 14.0, ff from http://packages.slackware.com/ (official repository). P.S. Complete cache flushing didn't help.

I would try the version of FF available on our site. https://www.mozilla.org/en-US/firefox/all/ (Note: I do not recommend using the 64-bit version at this time, even if your OS is 64-bit.)

If that doesn't work, try posting on some Slackware forums to see if anyone else on Slackware 14.0 is having this issue. Right now there haven't been any other reports of this, so I don't think it is affecting the more mainstream operating systems.

Wish I could offer more help.

Mark Schmidt said

I would try the version of FF available on our site. https://www.mozilla.org/en-US/firefox/all/ (Note: I do not recommend using the 64-bit version at this time, even if your OS is 64-bit.)

Same problem. It didn't help. Okay, I will ask in Slackware forums. Thank you very much and good luck!

Problem was in my router.

CPete said

Problem was in my router.

Hi CPete. I have the same problem and suspect could be the router. Something in the chain: Browser-->Ajax-->Router-->ISP-->PHP settings(php.ini, .htaccess)-->Server is wrong or has limits set. Can you explain in which sense the router was issue: - Did it work in other browsers and that router? - Did it work in Firefox with another router? Did you adjust/set router somehow? - Or it worked when you went around it, and had internet cable directly into pc?