Try to set the Boolean pref network.http.spdy.enabled to false on the about:config page.

• http://kb.mozillazine.org/about:config

Type about:config in the location (address) bar to open the about:config page and press the "Enter" key, just like you type the URL of a website to open a website.
If you see a warning then you can confirm that you want to access the about:config page.

• Use the Search bar at the top of the about:config page to locate preferences more easily
• Preferences that have been modified and do not have the default value show as bold (user set)
• Preferences can be reset to the default via the right-click context menu if they are user set
• Preferences can be changed via the right-click context menu: Modify (String or Integer) or Toggle (Boolean) or by double-clicking the line with the pref

So in this case you can type spdy.enabled in the search bar at the top of the about:config page.
You can double click that pref to toggle the value from true to false.
I don't know if a restart of Firefox is required (probably not), but you can close and restart Firefox to be sure.

You can check if it has helped and if not then you can undo this change and double click the pref another time or use Reset in the right-click context menu of that pref.

(in response of a PM)