Please see; https://support.mozilla.org/en-US/questions/1105959

Thanx FredMcD...I hope they fix the issue soon.

You can disable SPDY as a temporary workaround for Firefox 43 (it works in Firefox 44).

You can set this pref to false on the about:config page.

• network.http.spdy.enabled = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

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

You can update to Firefox 44, that should fix this issue.

• https://support.mozilla.org/kb/update-firefox-latest-version

If you have disabled SPDY then you can reset the SPDY pref on the about:config page.

• network.http.spdy.enabled = true