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 or otherwise make changes.

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

This can be a problem with the cache: wyciwyg stands for "What You Cache is What You Get"

"Clear the Cache": Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

You may have to delete the cache folder on disk to recover from this.
You can see the location on the about:cache page.

Hi,

  I wanted to acknowledge your prompt reply. As I stated in my original post and the "Troubleshooting" which I also filled out, I have had the problem on several computers with several different Windows OS's. I have tried every suggestion posted prior to today at Mozilla.org, on each of those computers.
  Just as did others whose posts were finally closed "4 months" ago.
  So I expect that I will merely "prove my point" by following your suggestions, which I do intend to carefully follow in the next 24 to 36 hours. 
  I will post the results, including "No Effect", as soon as I have a chance to try your suggestions. 
  Please keep in mind that the problem appears to be "intermittent" or at least, nobody has isolated any consistent causal relationship, even doesn't happen at the same URL all the time.
  You might want to re-read the closed post, which never provided a fix.

http://support.mozilla.org/en-US/questions/699151?page=1

  Again, thanks for your effort, I will soon post my results or lack thereof.

DrWattsOn ADDING Dec. 19, 2012 I decided to just work around it as described previously and accept that it won't be "fixed". Read my reply to replies below.

A possible solution appears here, suggesting why IE is immune to the problem http://www.experts-exchange.com/Q_27802818.html

@Fibo, Since I haven't really had the problem except as noted in my previous posts, I merely handle it as I also noted there.

  I do not play with javascript that much, though I get a sense of what it's trying to accomplish. But the link you provided goes to experts-exchange, where the "answer" is only available by starting a subscription with them, WHICH THEY WILL NEVER DELETE! And, their "Free" Trial requires the subscriber to proactively prevent them from automatic billing by "unsubscribing" which NEVER removes you from their database.
  I despise that business model. I have fallen for that elsewhere, so info on me may be everywhere, but I will not encourage yet another organization to add me to their database. I am not using hyperbole: I despise that business model. Just to sign up means they have your financial data forever. I despise the PEOPLE who do that.
  So, your reply provide me no solution, but rather, only a link to a "hook" claiming that they have the answer.

Have a look here, https://bugzilla.mozilla.org/show_bug.cgi?id=788547 solved the problem by removing old google analytics js code calling. Firefox as difficulties dealing with document.write() js function. Make sure your code doesn't include it without closing it.

WTF Mozilla !? Put my email somewhere else, I get enough spam like this !

The tone of your answer was uselessly aggressive. And proved that you do not know Expert-Exchange. You can have a free access to some answers each month, and if you contribute enough answers you have complete access for some period

Nevertheless, a more correct approach would have been something like "sorry but the link you provide is unavalable without registration", and I would havd provided the answer.

Being so aggressive to someone who proposes to help you, to implicitly suggest that this is business/ money related, is not the best way to find a solution.

But I guess you aready khew that.

Fibo, thank you for your reply. 1): "The tone of your answer was uselessly aggressive." Agreed, and I sincerely apologize. 2): "And proved that you do not know Expert-Exchange." Yes. And I had no way of knowing that upfront when I went to the link while dealing with a frustrating problem. Thank you for the info. 3): "... a more correct approach would have been something like ..." . I completely agree. 4): "Being so aggressive to someone who proposes to help you,..." agreed. 5): "... to implicitly suggest that this is business/ money related". I merely stated my opinion, as in 2) preceding, I had no way of knowing it was not when a "registration" was required. And I was being paranoid. Which does NOT mean they are NOT out to get me (or, my money)! 6): " ... is not the best way to find a solution. But I guess you aready knew that. " Well I should have and I appreciate your accurate retort. And again I apologize. And I thank you for taking time to fire back at me. I did need it on all the matters raised. I will try to be more laid back and wait until my immediate frustration wears off before posting. Some of my frustration was at how others asked the same questions and got basically the same answers, and no one's answer fixed my problem, nor was it fixed by Mozilla ... whom I do appreciate. As well as you, for getting back to me. Thanks, and I apologize for my lack of manners and civility. DrWattsOn PS: The matter has not arisen for many updates (now on 20.0.1) and basically was easy to work around: click in the Address field, then knock off everything but the preface wyciwyg etc.

Is there a site that reliably induces this problem for you? Some sites were mentioned in the other thread, but I don't know whether it's different for everyone.

jscher2000: It doesn't seem to happen anymore since several updates ago. As I recall, it only happened at my mailbox at yahoo, and it is unrelated to whether I had to log back on because I set a 2 week expiration on my local computer. It happened when selecting a tab that already had the last place I was at when at yahoo (usually the Inbox, and never the Log In page). As I said, I just dispensed with the preface in the Address bar.Hope this adds to the data helpfully.

The problem is caused by document.write not working in FF as the other browsers: in FF it should be used only at load time, not later.

So the problem has 2 solutions: - if you have no control over the javascript remove extra characters in the address bar - if this is your javascript, try to replace your document.write by creating a div with an id and use document.getElementById('the_id') += 'the_text'

or add document.close() after the doment.write() call.