Thanks JDPower - I have done that, too, in the past - but it always reverts. I do know there is an unresolved 'bug' (?) in Firefox which does not affect all users, and may depend on the other programs they are using. This causes followed links from some email programs to report that they cannot find the target of the link, when, in fact, Firefox does open up and find the target. Any work-arounds (like those given here) suggested in Firefox Help & Support, only work, generally, until the next Firefox update, when things revert. However, I know of know no other program, apart from CCleaner, which does what CCleaner does - that is, actually open the non-default browser, too.
I can live with this - it's really not at all important. But I do wonder how it actually happens, and whether - just as some web pages are sloppily coded (according to agreed, and now official, principles for coding web pages) but IE6 and IE7 will still open them 'normally', because the errors are ignored by those browsers, and a program like Firefox will not open them properly, because it does not allow such errors (which, doubtless, is one of the reasons it is a bit safer as a browser), so there might be a similar principle involved in the way web-links are written? Is the CCleaner link to its Update page 'correctly' coded?
Just an idle thought. Which is why I put it in this particular forum.