Can you please try opening firefox in safe mode and see if the issue still occurs? To do this, hold down the option key while you open firefox.

jgcoding said

I accepted the most recent update of Yosemite on my MacBook Pro this past weekend. Since then, Firefox is unusable, freezing within a few seconds after launch. This issue did not exist prior to the OSCX update. The freezing condition is so bad, I have to use the "force to quit" command from my launch pad. The browser is completely unresponsive and un-selectable otherwise. I have un-installed and re-installed FF several times and nothing works. I finally had to remove it permanently. I do not manage passwords or bookmarks but I do run about 8 add-ons, including a 3rd-party password and bookmark manager. I would like to get Firefox back on my machine please.

I did as instructed and navigated to the last offending page. The same thing happened. However, I had an idea. I forced quite and restarted in safe mode again and I went to drudgereport.com where I know no flash would be embedded. The browser did not freeze. So, I shut down FF and re-opened in normal mode for me (private) and loaded drudgereport.com again. The browser did not freeze. I followed a link to ABC news and the browser froze as their video tried to load. I had to force quit as before.

So, the issue may be with the Flash plug-in?

To test that theory, I disabled the "Shockwave Flash" add-on. I reloaded the original offending page. Sure enough, the page loaded and the browser did not freeze. I went back to the ABC news page, again the page did not freeze the browser with the "Shockwave Flash" add-on disabled.

So, it appears the issues is isolated to the "Shockwave Flash" add-on.