Thanks to all who responded to my question (see below). I followed your instructions to select the html attachment and then use the "Get Info" option to change not only that specific html attachment but all that are of the same type. It worked perfectly.
Thanks for helping me out. Ken Original message: I used PM 5.6.4 on a MacBook Pro running Leopard 10.5.2. Camino is currently my default browser. Things were fine until I had a problem with my laptop and ended up erasing the drive and restoring everything from a back-up clone. Now, even though Camino is my default browser (and is automatically opened and used at all other times), when a URL is sent as an attachment and I choose the attachment from within PM, it tends to open the web site using an ancient copy of Explorer that I have among my applications. I would appreciate any guidance about how I can adjust the settings so that PM always uses Camino (or whatever I choose as my computer's default browser). Thanks for your help. Ken
