I am running Firefox 38.1.1, but have also tried later versions with the same result. I have two desktop PCs, one a 32-bit and one a 64-bit, both running Windows 7. Earlier versions of Firefox worked as expected on both. Now everything works fine on the 32-bit, but on the 64-bit some sites (including Amazon.com, Facebook, and Firefox[!]) are displayed in text-only mode. This happens consistently, and re-booting does not fix the problem. I also uninstalled Firefox and re-installed, with the same result. I have not changed any settings, the problem simply occurred as soon as I updated from a previous version that was working correctly. Several 64-bit Firefox users at my workplace have reported the same problem, so it does not seem to be unique to my system. I am writing this bug report using Internet Explorer because I couldn't get here in Firefox. I much prefer Firefox to IE and hope you can fix this problem in a future release.
Hello Can you reproduce the issue in Safe Mode? If yes and you can reproduce the issue in Safe Mode, can you still reproduce it even with clean new fresh profile without any addons (extensions and plugins)? You can also try a standard troubleshoot and diagnose procedure.  = https://support.mozilla.org/kb/troubleshoot-firefox-issues-using-safe-mode  = https://support.mozilla.org/kb/refresh-firefox-reset-add-ons-and-settings  = https://support.mozilla.org/kb/troubleshoot-and-diagnose-firefox-problems P.S. Don't forget that 64bit Firefox is still not "stable" release.
I am running 32-bit Firefox (on a 64-bit PC), not 64-bit Firefox. The problem persists in safe mode and also after clearing all history. None of the "diagnose and fix" suggestions help. The problem seems to be one of security--Firefox is flagging Amazon.com and Facebook.com as untrusted sites.
I re-installed the privacy certificate for my own site and the problems have now disappeared. Mea culpa.
Based on comment 3.