I found that after Firefox v29, a LOT of my settings and
add-ons were changed / reset. Try this; Start Firefox in Safe Mode
While you are in safe mode;
Firefox Options > Advanced > General.
Look for and turn off Use Hardware Acceleration.
Clear the Cache
Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

and

Remove Cookies
Firefox/Tools > Options > Privacy > Cookies: "Show Cookies"

Then check ALL of your settings. Browser and add-ons. Then restart.

You can check the Net log and the Network log in the Web Console (Firefox/Tools > Web Developer) to see what content is blocked and has a zero file size.

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• Hold down the Shift key and left-click the Reload button
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Command + Shift + R" (Mac)

Clear the cache and cookies only from websites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes