Odd hyperlink behaviour in Firefox

I wonder if anyone else is experiencing problems with hyperlinks in webhelp running on Firefox?

I have created a new webhelp system - nothing clever such as merging - just 70 or so topics.The help contains hyperlinks to other topics in the webhelp - these are left to page default (the intention being that they should open and replace the current page).

I have published the help to my C drive and am running it from there. When I select a link to another topic, the target topic opens for a second, then the help jumps to the parent folder for that topic. I just tried another webhelp system that I created a week ago (and was fine then) and it shows the same behaviour.

When I do exactly the same in IE, it works as expected.

I'm using Robohelp 7, Firefox 3.6 (it was the same with Firefox 3.5.7 so  I tried upgrading) and Vista (though I tried on a Mac and it's the same, so I've ruled out an operating system quirk).

I'm guessing this is down to a recent Firefox update...or have I just done something daft?