In the end, my trial of Firefox 3.0 didn't outlast the first week. I could have lived with it if it hadn't futzed up my right clicking of folders in the Bookmark panel. I DID discover that All-In-One Sidebar was responsible in some way for the problem. But even without AIO Sidebar, the default behaviour of right-clicking was all wrong. Instead of opening at the far right of the queue of tabs, it opened up to the immediate right of the currently-selected tab ... if it didn't replace that tab along the way.
BUT, maybe things will get fixed next week. Sometime next week (the target is Tuesday, July 15th), they will be releasing the update to v3.0. There will be the usual security fixes in the product, but there will also be bug fixes. I hope the issue I hold nearest and dearest to my right thumb will be amongst them.
I WILL give v3.01 a try again and report here.