Re: When opening firefox, it always says 500 internal server error.
This worked for me. It seemed one of my add-ons called "Relevant Knowledge" was the culprit. I'm not even sure what this add-on is or when I even installed it. But it doesn't appear to be necessary as Firefox is working fine for me now.
I suggest you download Malware Bytes which has saved my butt a few times. Reason being, I've also seen a few threads which suggest you must remove this from your registry. Perform a quick scan firt. Then full later. I'm running one now and it's alredy found a couple (28) of issue from this "Relevant Knowledge Spyware."
Also, this is how it was installed on my machine. I downloaded a "Free Flash Video Converter" from LEAWO.COM and installed their software. That's when it was placed onto my cpu. I can verify that 2 other machines in my office were infected when downloading this software from their company. Advice is to stay away http://www.leawo.com until they shape up. Their software was great though.
Note: One employee when downloading, was prompted by her anti virus that something was amiss.