Many English web pages filled with meaningless Chinese characters - why?

Question

I've been using web browsers for almost 20 years and this is one of the strangest problems I've encountered. When some Firefox pages are launched that are supposed to be straight forward English language pages, they are displayed as an entire page densely filled, top to bottom, full width, with tiny Chinese characters. I have tried to troubleshoot this by: disabling all add-ons, especially translators; clean uninstall/reinstall of the current version of Firefox; rolling back to previous versions of Firefox - 1st FF7, right now FF6.01; clean uninstall/reinstall of my graphics card driver; rolling back to previous versions of my graphics card driver; system restore back to before this problem started; run deep scans, multiple times, of each of the six anti-malware apps I have on my computer, including root kit scans. None of them have solved the problem.
Strangest part of it is that it seems to occur randomly. For example, in my latest Firefox session, I re-enabled a number of add-ons after disabling them as part of my ongoing troubleshooting. When I clicked on Options for all the others except the Groowe toolbar, I got normal English pages. But for Groowe toolbar options I received a full page of Chinese characters like those shown below.This happens pretty much every time Firefox session but not for all pages; only 1-3 of them per session. The rest are normal.
I have yet to detect a pattern of when it will happen. A given web page may display normally or it may display as all Chinese characters like this sample of the page just displayed for Groowe Options. The last time I set Groowe Options (= the last time I reinstalled Firefox - yesterday) it displayed normally.
This paste of the first few lines of the Groowe Options page is a bigger font than the one displayed on the web page it was copied from. I don't know how to make it paste the same size font, but I think you'll get the picture from this. Now just imagine tiny versions of these lines completely covering the page display -- and I do mean completely - no spaces, no blank lines, just non-stop characters like this but much smaller font.
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Answer 1

The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!)<br>
http://www.joelonsoftware.com/articles/Unicode.html<br>