The bug appeared when working with blogs and content management systems. As cernio wrote as a follow-up to my initial post, The original abbeyphone Firefox VoIP Extension is putting some garbage in nearly all the web pages which contain user numbers as user input: therefore, any application that contains a <textarea> tag may be affected by this bug.
This was roughly similar to what Skype does to enable a phone number to be dialed directly from a web page. Unfortunately, though, that when the pages I was creating were opened in Firefox, script for click-to-call produced visible prompts to do just that- even though click-to-call was not intended.
Now, with The Firefox and Thunderbird extensions added to the newest, abbeyphone rev, this won't happen any more.
And that's a good thing.