Perform the suggestions mentioned in the following articles:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems
• Update All your Firefox Plugins -> https://www.mozilla.org/en-US/plugincheck/
• When Downloading Plugins Update setup files, Remove Checkmark from Downloading other Optional Softwares with your Plugins (e.g. Toolbars, McAfee, Google Chrome, etc.)

Check and tell if its working.

I was encountering a similar problem with Twitter and Tumblr login pages, as seen here. It turned out to be Kaspersky for Mac and its associated plugin for FF stopping the scripts from loading correctly.

Do you have this Anti-virus program or similar? If so, try disabling any FF add-on associated with it and restart FF.